MB-11325: Add a stat showing what seqno a cursor is pointing to

This will help with debugging upr related issues.

Change-Id: I4e7735a5c75eed1806d3b1577b35190b2e17c112
Reviewed-on: http://review.couchbase.org/38018
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>