Call sync.h primitives "locks" and "mutexes" instead of "blocks" and "waitable critical sections" to match current coding conventions and c++11 standard names.

This PR does not rename the "CCriticalSection" class (though this could be done as a followup) because it's used everywhere and would swamp the other changes in this PR. Plain mutexes should mostly be preferred instead of recursive mutexes in new code anyway.