we should reduce the frame buffer size in NewWebsocket

[John A Meinel]

For compatibility with clients that didn't support framing, when we introduced the new gorilla websockets, we used 64kB buffers. However, that is now a significant source of memory overhead.

Since gorilla supports message frames, and we no longer need to worry about 2.1 agents/clients against a new controller, we should be able to drop the buffers back to their default 4096 bytes.

We need to do cross-version testing with reasonably large messages to convince ourselves that the frames are working correctly, but that should reduce one of our primary sources of memory consumption.

[John A Meinel (jameinel)]

https://github.com/juju/juju/pull/9701 is a patch against 2.5. We may go for 2.5.1 or we may delay for 2.5.2