Collections: Add scope support to GetAllVBSequenceNumbers

A DCP stream can be for the bucket, a scope or a collection (actually can be custom set of collections).

For DCP streams against a bucket or single collection, GetAllVBSequenceNumbers (which is command orginally added for DCP) it can be encoded in a way to request for the bucket or the single collection, however there is a gap in that there is no way to ask for the scope, the client would have to figure that out with many round-trips (which could be troublesome if the scope is changing concurrently).

This ticket tracks that the server should plug that gap and provide a way so the client can ask about a scope, allowing the server to return the max-seqno of the scope in one request.

This ticket does not cover a solution for the multi-collection stream.