keep them open, but do a websocket ping/pong to check if the
connection is alive.
This is usually handled by browsers themselves, but if you are using
websockets from a non-browser then you may have to PONG back on
a PING, see https://www.rfc-editor.org/rfc/rfc6455#section-5.5.2
(note that PING-PONG is a requirement there)
Eg if there are 10.000 users online and you do user.list.
The old websocket framing assumed no response was >64Kb.
This also creates a new function websocket_create_packet_ex()
This makes websocket_common unload last (and near-last: rpc & websocket)
and makes us call Mod_Init for these three modules first.
This way, the period where the websocket handler is unavailable is kept
to a minimum.
This also renames the ModuleSetOptions option MOD_OPT_UNLOAD_PRIORITY
to MOD_OPT_PRIORITY since it dynamically changes the module priority
in the list. For 6.x compatibility, MOD_OPT_UNLOAD_PRIORITY can still
be used.
Bram Matthys