MB-19678: Merge backfill and in-memory snapshots correctly on replica vb

MB-19678: Merge backfill and in-memory snapshots correctly on replica vb

When a DCP client, on replica vb, opens a stream which it intends to
keep open forever, merge the backfill and in-memory snapshots by using the
the checkpoint snapshot_end as snapshot_end_seqno.

Change-Id: Ic05a59ccafa54bbee19882707404a78c47002be7
Reviewed-on: http://review.couchbase.org/64207
Well-Formed: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Manu Dhundi <manu@couchbase.com>