Enable flow control for intra-cluster replication by default

We have fixed all know issues and run a bunch of tests to ensure
that this is now working. As a result we will turn flow control
back on by default.

Change-Id: Ib3a498b9798eb7eccf03722464a220dd3f7da828
Reviewed-on: http://review.couchbase.org/38367
Reviewed-by: Sriram Ganesan <sriram@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>