MB-10348: Vbucket's purge_seqno available in stats

vbucket-seqno will list 3 stats now:
- vb_x:uuid
- vb_x:high_seqno
- vb_x:purge_seqno

Requires: http://review.couchbase.org/#/c/35062/

Change-Id: Icb501b2d305c5ee1efb3903f8c166ccb986c1c61
Reviewed-on: http://review.couchbase.org/35035
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: buildbot <build@couchbase.com>