Collections: backfill from collection start-seqno

This has been a long standing idea and its mileage may vary. The idea is that if a client does not use OSO backfill (which is still optional at this time) and they do a collection DCP stream (start=0) KV should adjust the start-seqno to be the start of the requested collection (the start-seqno is part of the collection's meta-data).

This can result in a reduction in I/O as we will not be reading parts of the index which are not for the requested collection.

If the DCP request is for a scope or set of collections, we adjust the start to the lowest seqno of the scope/set.

Note: If we make OSO mandatory for collection streams, this technique becomes less important.