upr client: add fake producer support for remove_streams and pausing

For testing multiplexed streams, it requires additionaly support
from the fake producer to slow down the sending of mutations. Helpers
APIs has been added to pause and continue mutations to the client from
fake server. Support for stream_close message also has been added.

Change-Id: I6a2aad3cf28555609a6344128f9c8bcd36d3f538
Reviewed-on: http://review.couchbase.org/34233
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>