MB-11259: send window update size in ext field

Mike said it's because we usually send fixed-size things in ext and
body is usually for variable-sized things.

This depends on corresponding memcached commit here:
http://review.couchbase.org/37746

Change-Id: I727e8febce13dbd6289823df1e6389df4bdacf2f
Reviewed-on: http://review.couchbase.org/38767
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>