upr client: add tests for multiplexing and remove streams

Added unit tests for multiplexed vbucket streams, listing
active streams and remove streams API.

Change-Id: I0e337e66635f5a69cb3aa654aaf5b07205ebb98e
Reviewed-on: http://review.couchbase.org/34234
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>