Only lock in map update timer when available.

Only lock the mutex cs_main in the UI map update timer when the lock is
immediately available and skip update ticks if this is not the case.
Merge branch 'master' into uiupdate-trylock
Merge branch 'master' into uiupdate-trylock
Merge pull request #13 from domob1812/uiupdate-trylock

Reduce UI freezes via conditional lock