Improve slasher cold boot duration.

Before this commit, on a slasher cold boot (aka, without any db),
the `updatedChunkByChunkIndex` function looped for all validators
AND for all epochs between the genesis epoch and the current epoch.

This could take several dozen of minutes, and it is useless since the
min/max spans are actually a circular buffer with a limited lenght.
Cells of min/max spans can be overwritten (with the same value)
plenty of times.

After this commit, the `updatedChunkByChunkIndex` function loops
for all validators AND AT most 'historyLength' lenght.
Every cell of min/max spans are written AT MOST once.

Time needed for slasher boot goes from `O(nm)` to "only" `O(m)`, where:
- `n` is the number of epochs since the genesis.
- `m` is the number of validators.

beacon-chain/slasher/detect_attestations.go

@@ -247,8 +247,6 @@ func (s *Service) updatedChunkByChunkIndex(
 	currentEpoch primitives.Epoch,
 	validatorChunkIndex uint64,
 ) (map[uint64]Chunker, error) {
-	var err error
-
 	chunkByChunkIndex := map[uint64]Chunker{}
 
 	validatorIndexes := s.params.validatorIndexesInChunk(validatorChunkIndex)
@@ -257,11 +255,21 @@ func (s *Service) updatedChunkByChunkIndex(
 		latestEpochWritten, ok := s.latestEpochWrittenForValidator[validatorIndex]
 
 		// Start from the epoch just after the latest epoch written.
-		epochToWrite := latestEpochWritten + 1
+		epochToWrite, err := latestEpochWritten.SafeAdd(1)
+		if err != nil {
+			return nil, errors.Wrap(err, "could not add 1 to latest epoch written")
+		}
+
 		if !ok {
 			epochToWrite = 0
 		}
 
+		// It is useless to update more than `historyLength` epochs, since
+		// the chunks are circular and we will be overwritten at least one.
+		if currentEpoch-epochToWrite >= s.params.historyLength {
+			epochToWrite = currentEpoch + 1 - s.params.historyLength
+		}
+
 		for epochToWrite <= currentEpoch {
 			// Get the chunk index for the latest epoch written.
 			chunkIndex := s.params.chunkIndex(epochToWrite)

beacon-chain/slasher/detect_attestations_test.go

@@ -1015,6 +1015,39 @@ func Test_epochUpdateForValidators(t *testing.T) {
 				0: {neutralMax, neutralMax, neutralMax, 55, neutralMax, neutralMax, neutralMax, 55},
 			},
 		},
+		{
+			name:                               "start with some data - high latest updated epoch",
+			chunkSize:                          4,
+			validatorChunkSize:                 2,
+			historyLength:                      12,
+			currentEpoch:                       16,
+			validatorChunkIndex:                21,
+			latestUpdatedEpochByValidatorIndex: map[primitives.ValidatorIndex]primitives.Epoch{42: 2, 43: 3},
+			initialMinChunkByChunkIndex: map[uint64][]uint16{
+				// | validator 42 |  validator 43 |
+				0: {55, 55, 55, 55, 55, 55, 55, 55},
+				1: {66, 66, 66, 66, 66, 66, 66, 66},
+				2: {77, 77, 77, 77, 77, 77, 77, 77},
+			},
+			expectedMinChunkByChunkIndex: map[uint64][]uint16{
+				// |                  validator 42                |                  validator 43                 |
+				0: {neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin},
+				1: {neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin},
+				2: {neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin, neutralMin},
+			},
+			initialMaxChunkByChunkIndex: map[uint64][]uint16{
+				// | validator 42 |  validator 43 |
+				0: {55, 55, 55, 55, 55, 55, 55, 55},
+				1: {66, 66, 66, 66, 66, 66, 66, 66},
+				2: {77, 77, 77, 77, 77, 77, 77, 77},
+			},
+			expectedMaxChunkByChunkIndex: map[uint64][]uint16{
+				// |                  validator 42                |                  validator 43                 |
+				0: {neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax},
+				1: {neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax},
+				2: {neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax, neutralMax},
+			},
+		},
 	}
 
 	for _, tt := range testCases {