Allow Sync Service to Query State Of Network

[nisdas]

Resolves #555, #661, #743

This allows the sync service to query peers in the network to find out their current head and determine if they are synced or not.

• Implement IsSynced method, which will allow the node to know whether to begin initial sync or normal sync.
• Allow the node to access the current db instead of requiring a new one, so that they know where to sync from.
• Add tests .

[codecov]

Codecov Report

Merging #787 into master will decrease coverage by 0.23%.
The diff coverage is 58.67%.

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
- Coverage   73.65%   73.41%   -0.24%     
==========================================
  Files          66       67       +1     
  Lines        4217     4322     +105     
==========================================
+ Hits         3106     3173      +67     
- Misses        807      841      +34     
- Partials      304      308       +4

[prestonvanloon]

I'm experiencing some issues when running the command three times.

Run the following command for about 15 seconds then kill the process with ^C.

bazel run //beacon-chain -- --datadir=/tmp/data1 --rpc-port 4000 --genesis-json=$(pwd)/genesis.json --simulator --demo-config

Do this twice, on the third attempt, you will see the follow errors:

[2018-11-18 12:36:56] ERROR types: invalid attestation slot 13
[2018-11-18 12:36:56] ERROR types: Attestation invalid: slot:13 shard:4 justified_block_hash:"\323\003[v\005X\267\242jp`\326V\205\r\025\256\361\241\270\005r\360\236\262*5\257J\223-\306" attester_bitfield:"\300"
[2018-11-18 12:36:56] ERROR types: Invalid attestation
[2018-11-18 12:36:56] ERROR blockchain: block failed validity conditions
[2018-11-18 12:36:58] ERROR blockchain: block points to nil parent: 0x9d8b46a5257cf03279f2263587aa53542c65b50d8e00647a48c68a3ec97b135e
[2018-11-18 12:37:00] ERROR blockchain: block points to nil parent: 0xc3c1e41a63fcd7649861969745f96ee58bffeb840b8e2189783e3008ff719d3c

On the 4th attempt, I see

[2018-11-18 12:43:38] ERROR simulator: Failed to get shard committee: slot 214 out of bounds: 5 <= slot < 20
[2018-11-18 12:43:40] ERROR simulator: Failed to get shard committee: slot 215 out of bounds: 5 <= slot < 20
[2018-11-18 12:43:42] ERROR simulator: Failed to get shard committee: slot 216 out of bounds: 5 <= slot < 20
[2018-11-18 12:43:44] ERROR simulator: Failed to get shard committee: slot 217 out of bounds: 5 <= slot < 20
[2018-11-18 12:43:46] ERROR simulator: Failed to get shard committee: slot 218 out of bounds: 5 <= slot < 20

These might be out of scope, but we should identify issues for these if we haven't already.

[prestonvanloon]

DIscussed offline that the issues I outlined are addressed in the other pull request #669.

Thanks!