CBG-3703 create a basic bucket to bucket rosmar implementation

[torcolvin]

This implements the features of standard XDCR without mobile setting. Followup tickets will address handling of _vv and _mou entries.

This was reimplemented from #6746 with the same code but moved entirely into one repo. The rationale for moving into a single repo is to make modifications easier to work with. All code in xdcr package should be test-only code and we could name the package as such to reconcile that.

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/2378/