websocket / pjsip: Increase maximum packet size.

When dealing with a lot of video streams on WebRTC
the resulting SDPs can grow to be quite large. This
effectively doubles the maximum size to allow more
streams to exist.

The res_http_websocket module has also been changed
to use a buffer on the session for reading in packets
to ensure that the stack space usage is not excessive.

Change-Id: I31d4351d70c8e2c11564807a7528b984f3fbdd01