MB-11107: Give upr flow control stats more descriptive names

When debugging a flow control issue I often found myself confused
as to what some of the stats really meant. Changing the names to
be more descriptive helped me to more quickly debug issues.

Change-Id: I079b79bb6c92d7ec7b5c61c3a3aa5ad888b6d979
Reviewed-on: http://review.couchbase.org/37624
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>