MB-11416 Make control buffer size configurable for QE tests

Make buffer size configurable, so that QE can configure it to lower
value and verify that unacked_bytes are less then 20 % of buffer size.

Change-Id: I75fd8bbcc4333916bd6cfb09c76b3432553fddb8
Reviewed-on: http://review.couchbase.org/38227
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>