Beacon node slasher improvement (#13549)

* Slasher: Ensure all gorouting are stopped before running `Stop` actions.

Fixes #13550.
In tests, `exitChan` are now useless since waitgroup are used to wait
for all goroutines to be stopped.

* `slasher.go`: Add comments and rename some variables. - NFC

* `detect_blocks.go`: Improve. - NFC

- Rename some variables.
- Add comments.
- Use second element of `range` when possible.

* `chunks.go`: Remove `_`receivers. - NFC

* `validateAttestationIntegrity`: Improve documentation. - NFC

* `filterAttestations`: Avoid `else`and rename variable. - NFC

* `slasher.go`: Fix and add comments.

* `SaveAttestationRecordsForValidators`: Remove unused code.

* `LastEpochWrittenForValidators`: Name variables consistently. - NFC

Avoid mixes between `indice(s)`and `index(es)`.

* `SaveLastEpochsWrittenForValidators`: Name variables consistently. - NFC

* `CheckAttesterDoubleVotes`: Rename variables and add comments. - NFC

* `schema.go`: Add comments. - NFC

* `processQueuedAttestations`: Add comments. - NFC

* `checkDoubleVotes`: Rename variable. - NFC

* `Test_processQueuedAttestations`: Ensure there is no error log.

* `shouldNotBeSlashable` => `shouldBeSlashable`

* `Test_processQueuedAttestations`: Add 2 test cases:
- Same target with different signing roots
- Same target with same signing roots

* `checkDoubleVotesOnDisk` ==> `checkDoubleVotes`.

Before this commit, `checkDoubleVotes` did two tasks:
- Checking if there are any slashable double votes in the input
  list of attestations with respect to each other.
- Checking if there are any slashable double votes in the input
  list of attestations with respect to our database.

However, `checkDoubleVotes` is called only in
`checkSlashableAttestations`.

And `checkSlashableAttestations` is called only in:
- `processQueuedAttestations`, and in
- `IsSlashableAttestation`

Study of case `processQueuedAttestations`:
---------------------------------------------
In `processQueuedAttestations`, `checkSlashableAttestations`
is ALWAYS called after
`Database.SaveAttestationRecordsForValidators`.

It means that, when calling `checkSlashableAttestations`,
`validAtts` are ALREADY stored in the DB.

Each attestation of `validAtts` will be checked twice:
- Against the other attestations of `validAtts` (the portion of
  deleted code)
- Against the content of the database.

One of those two checks is redundent.
==> We can remove the check against other attestations in `validAtts`.

Study of case `Database.SaveAttestationRecordsForValidators`:
----------------------------------------------------------------
In `Database.SaveAttestationRecordsForValidators`,
`checkSlashableAttestations` is ALWAYS called with a list of
attestations containing only ONE attestation.

This only attestaion will be checked twice:
- Against itself, and an attestation cannot conflict with itself.
- Against the content of the database.

==> We can remove the check against other attestations in `validAtts`.

=========================

In both cases, we showed that we can remove the check of attestation
against the content of `validAtts`, and the corresponding test
`Test_checkDoubleVotes_SlashableInputAttestations`.

* `Test_processQueuedBlocks_DetectsDoubleProposals`: Wrap proposals.

So we can add new proposals later.

* Fix slasher multiple proposals false negative.

If a first batch of blocks is sent with:
- validator 1 - slot 4 - signing root 1
- validator 1 - slot 5 - signing root 1

Then, if a second batch of blocks is sent with:
- validator 1 - slot 4 - signing root 2

Because we have two blocks proposed by the same validator (1) and for
the same slot (4), but with two different signing roots (1 and 2), the
validator 1 should be slashed.

This is not the case before this commit.
A new test case has been added as well to check this.

Fixes #13551

* `params.go`: Change comments. - NFC

* `CheckSlashable`: Keep the happy path without indentation.

* `detectAllAttesterSlashings` => `checkSurrounds`.

* Update beacon-chain/db/slasherkv/slasher.go

Co-authored-by: Sammy Rosso <15244892+saolyn@users.noreply.github.com>

* Update beacon-chain/db/slasherkv/slasher.go

Co-authored-by: Sammy Rosso <15244892+saolyn@users.noreply.github.com>

* `CheckAttesterDoubleVotes`: Keep happy path without indentation.

Well, even if, in our case, "happy path" mean slashing.

* 'SaveAttestationRecordsForValidators': Save the first attestation.

In case of multiple votes, arbitrarily save the first attestation.
Saving the first one in particular has no functional impact,
since in any case all attestations will be tested against
the content of the database. So all but the first one will be
detected as slashable.

However, saving the first one and not an other one let us not
to modify the end to end tests, since they expect the first one
to be saved in the database.

* Rename `min` => `minimum`.

Not to conflict with the new `min` built-in function.

* `couldNotSaveSlashableAtt` ==> `couldNotCheckSlashableAtt`

---------

Co-authored-by: Sammy Rosso <15244892+saolyn@users.noreply.github.com>

beacon-chain/db/slasherkv/schema.go

@@ -7,10 +7,21 @@ package slasherkv
 // it easy to scan for keys that have a certain shard number as a prefix and return those
 // corresponding attestations.
 var (
-	// Slasher buckets.
-	attestedEpochsByValidator  = []byte("attested-epochs-by-validator")
-	attestationRecordsBucket   = []byte("attestation-records")
+
+	// key: (encoded) ValidatorIndex
+	// value: (encoded) Epoch
+	attestedEpochsByValidator = []byte("attested-epochs-by-validator")
+
+	// key: attestation SigningRoot
+	// value: (encoded + compressed) IndexedAttestation
+	attestationRecordsBucket = []byte("attestation-records")
+
+	// key: (encoded) Target Epoch + (encoded) ValidatorIndex
+	// value: attestation SigningRoot
 	attestationDataRootsBucket = []byte("attestation-data-roots")
-	proposalRecordsBucket      = []byte("proposal-records")
-	slasherChunksBucket        = []byte("slasher-chunks")
+
+	// key: Slot+ValidatorIndex
+	// value: (encoded) SignedBlockHeaderWrapper
+	proposalRecordsBucket = []byte("proposal-records")
+	slasherChunksBucket   = []byte("slasher-chunks")
 )