CBG-4420: fix legacy rev handling on processRev

[gregns1]

CBG-4420

• Add code to pull rev tree from history if it exists
• Add conflict check fro rev tree history if that exists on rev message
• Add tests for conflict and non conflict scenarios
• Add tests for each test case that we can from design doc (some require changes to blip test client)

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/000/