Use "\0" instead of "|" as a push message delimiter (#11692)

Used with TrackMessageSizeInterceptor and with client-to-server Websocket message splitting (see #11648)

The original issue that the delimiter can not appear in the message (unescaped) is apparently fixed in Atmosphere 1.0.13

Also ensure the max size of a websocket fragment in bytes does not exceed the buffer size (#11842)

Change-Id: I768524bb54a5b8b9479dc7bda821256bd843dc52