Return in-memory snapshot as well

Prior to this change every snapshot of the UPR fake server was
returning snapshots flagged as on-disk snapshots. With this change
only snapshots that start from sequence 0 are considered to be
on-disk snapshots, all others are in-memory snapshots.

Change-Id: Ia3f78c7caacd002d62689be8b3fe0b4c7a10eec0
Reviewed-on: http://review.couchbase.org/37123
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>