CBQE-2284: flowcontrol to testrunner uprclient

For setting flow control per upr_client connection.
General usage is to:
1) set buffer size
upr_client.flow_control(128)

2) send a stream request
upr_client.stream_request(...)

3) ack unacked data while fetching mutations
upr_client.ack(unacked)

Change-Id: Ifa3d3e6af05b91e732369d2ff42eb4e2c9500a4a
Reviewed-on: http://review.couchbase.org/37861
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>