Add 'stats vbucket-seqno' command

This command lists all of the high sequence numbers for all
vbuckets.

Change-Id: Ie007d9549857a2b485d7df43931be23a04b2c23c
Reviewed-on: http://review.couchbase.org/30778
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>