MB-11005 Lower send/receive buffer sizes for replicators.

Also make it configurable.

Change-Id: I806b9302175f1abfc87b3e9fe093c4ff989a35b6
Reviewed-on: http://review.couchbase.org/39001
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>