On hitting max pending updates to rabbit (via configurable knob rabbit_max_pending_updates) refuse to do stateful changes (put/post/del)
Add unit test for this.

Change-Id: Ia7965626d840adda23dc16903a8afe1556bd1f83
(cherry picked from commit f2e838155532e8a4cfc219bc66a9d8ab62e0e89b)