upr client: add tests for stream events queue flow control

Change-Id: I8c66d17c0d1595cd13cd39e34813e29afafd0e0a
Reviewed-on: http://review.couchbase.org/34239
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>