Today, due to how netty works (both on http layer and transport layer), and even though the buffers sent over to netty are paged (CompositeChannelBuffer), it ends up re-copying the whole buffer into another heap buffer (bad), and then send it over directly to sun.nio which allocates a full thread local direct buffer to send it (which can be repeated if not all message is sent).
  This is problematic for very large messages, aside from the extra heap temporal usage, the large direct buffers will stay around and not released by the JVM.
  This change forces the use of gathering when building a CompositeChannelBuffer, which results in netty using the sun.nio write method that accepts an array of ByteBuffer (so no extra heap copying), and also reduces the amount of direct memory allocated for large messages.
  See the doc on NettyUtils#DEFAULT_GATHERING for more info.
closes elastic#7811