CBG-4321: store skipped sequences as single entries below a certain threshold

[gregns1]

CBG-4321

• Define a new threshold for what skipped seqs need to meet to be stored as a range on the skipped list
• If below this threshold we will push skipped entries as single entries
• Changes to tests for this change

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2762/

[adamcfraser]

This is going to result in a separate lock acquisition for each sequence in the range. Perhaps we want a PushSkippedSequenceEntries(startSeq, endSeq) option on SkippedSequenceSlice that gets the lock once? That would also let us append the set of entries to the list in a single append operation, if that turns out to be more efficient.