Consolidate MutexLockerEx and MutexLocker

MutexLockerEx is almost identical to MutexLocker, except that it allows NULL and to pass a safepoint checking parameter. These two branches seem like an uninteresting distinction for minimal or likely no performance benefit. The operation takes a lock which will dwarf any predicted branches.

Also, there's a corresponding MutexUnlocker/MutexLockerEx which seemes even less necessary for performance.