MB-9585: Add support for upr stats

This change refactors the TAP stats related functions and added
UPR support. Client such as cbstats should be able to retrieve
"upr" or "upragg" stats. We'll add comprehensive stats for UPR
later on. Also modified some upr test cases that depends on stats.

Change-Id: I1526c37700a11251d6bbcb8eb4d253e12455a722
Reviewed-on: http://review.couchbase.org/30855
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>