Change the flow control buffer to 10MB

The original size of 2MB is too small and causes rebalanace to
become slow. We should increase the buffer to 10MB by default
in order to speed things up. Note that the amount of consumer
connections on a node will be small so this should be okay.

Change-Id: I2804f122dc5b39b0d0229aa8dc87964d711e96b4
Reviewed-on: http://review.couchbase.org/37919
Reviewed-by: David Liao <david.liao@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>