MB-11107: Add an acked_bytes stat to all upr consumers and upr producers

These stats will help with debugging upr flow control issues.

Change-Id: I58b93fc77ac4271d4072a7a27812740b500bca3a
Reviewed-on: http://review.couchbase.org/37625
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>