Add asynchronous pending item count

This replaces the older individual hashset_t's allocated for the
instance. Items which should block the loop are now inserted via an
opaque API.

Change-Id: Ic9bf079a138c104b874bf2fc0062986940a2c810
Reviewed-on: http://review.couchbase.org/36571
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>