* Minimal: Explicit log levels
* Minimal: Formspec test now disabled by default
This is done because the formspec test was disruptive.
This test can now be toggled with server command test_formspec first.
* Add minetest.bulk_set_node call + experimental mod unittest
* Optimize set_node function to prevent triple lookup on contentfeatures
Do only one lookup for old, and try to merge old and new lookup if node is same than previous node
* Add benchmark function + optimize vector population to have real results
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
On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.
Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)
