Decrease the websocket buffer size due to a Jetty 9.1 issue (#13087)

Jetty 9.1 throws if InputBufferSize is set to greater than or equal to
MaxTextMessageBufferSize. We cannot simply increase the value of the latter
because Atmosphere sets the former first. Thus, its value must be set to
less than 32768 (the default for MaxTextMessageBufferSize). This should not
cause problems with regard to performance; the original figure of 65536
was more or less an arbitrary choice.

Change-Id: If9596fc2bffdd14e8c1f31ff4b9b10d6685e01ed