[3.0.1 backport] CBL revpos handling causes attachment fetch per write for docs with attachments

When pushing a mutation to a document with an attachment, CBL is setting the attachment revpos to the generation of the current revid, instead of the revision at which the attachment is added.

This results in Sync Gateway treating the attachment as new to Sync Gateway, and results in a Sync Gateway fetching the attachment from Couchbase Server in order to generate a proveAttachment request.

This will create a lot of unnecessary network traffic between SG and CBS for highly mutated documents with attachments.  

In cases where the attachment is already present with matching digest on the common ancestor of the incoming revision and the active revision, proveAttachment should not be performed, regardless of revpos.