The default size is set to 8KB which can be easily exceed even by batch
of 100 flows. When this happens ONOS closes the channel as consequence
of the HTTP2Exception thrown by Netty. With this patch we set the size
to 40MB which should avoid any problem even with batch larger than 100k
flows.
Change-Id: I3f1ccbf20275898798e039531d1871991d99d952