upr client: add support for throttling events queue

The upr client internally needs to buffer a limited
amount of mutations specified by max_buffer_size. If the
reader is slow, the client internally stops reading from
tcp socket and makes use of tcp flow control.

Change-Id: If6ac657987049891d963f8c21bcee86372aa4c0b
Reviewed-on: http://review.couchbase.org/34238
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>