CBG-4254 implement Sync Gateway peer

[torcolvin]

Depends on https://github.com/couchbase/sync_gateway/pull/7158, this is marked to merge there.

Since we need to do APIs using revtreeids for optimistic locking, but the REST api doesn't support using them, I decided to use db/crud.go APIs directly rather than parsing REST endpoints.

Pre-review checklist

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