upr client: Allow only single waiter per vbucket stream

Restrict upr client to allow only one event_event waiter
per vbucket stream. Added a unit test for this condition
as well.

Change-Id: I76568fe84c6260dd554f7b80e5d4d49128b8c6e9
Reviewed-on: http://review.couchbase.org/34646
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>