MB-8244 add EXECUTOR_WAITING & EXECUTOR_SLEEPING

This new states will be useful for indicating which
exucutor thread (readers/writers) is waiting. This
will be available as part of readers/writers stat.

Change-Id: Id6179855ca2d430bc7d1356b40e231a1506be2b5
Reviewed-on: http://review.couchbase.org/27038
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Jin Lim <jin@couchbase.com>