MB-10304 Implementation of stream multiplexing upr client

Implemented a gen_server similar to existing couch_upr gen_server
implementation providing same APIs. The existing couch_upr test has
been modified to use multiplexing client.

Change-Id: Ie74c926f5c3e2d87d7b37e8cdac803225fef552a
Reviewed-on: http://review.couchbase.org/33926
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>