Implement mod communication channels (#6351)

Implement network communication for channels

* Implement ModChannel manager server side to route incoming messages from clients to other clients
* Add signal handler switch on client & ModChannelMgr on client to handle channels
* Add Lua API bindings + client packet sending + unittests
* Implement server message sending
* Add callback from received message handler to Lua API using registration method

games/minimal/mods/experimental/init.lua

@@ -2,6 +2,8 @@
 -- Experimental things
 --
 
+dofile(minetest.get_modpath("experimental").."/modchannels.lua")
+
 -- For testing random stuff
 
 experimental = {}