Mod profiling support

Config settings:
profiling = true/false (gather mod statistics)
detailed_profiling = true/false (break mod times to callbacks)

Chat commands:
save_mod_profile saves current statistics in debug.txt and shows on console (on default loglevel)

Example what you're gonna see:
https://gist.github.com/sapier/4629aa2acfa1b943df97
<code>-- Minetest: builtin/game/mod_profiling.lua</code>