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

Change-Id: I01e021f7aba63aa9144b88f3eba62a52a0517083
Reviewed-on: http://review.couchbase.org/35202
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: buildbot <build@couchbase.com>