Clean up Monitor code

[kouvel]

Examples:

• See comments in Improve Monitor scaling coreclr#14216
• Refactor EnterObjMonitorHelper and EnterObjMonitorHelperSpin to simplify them
• Move AwareLock-relevant logic into member functions of AwareLock
• Consider adding a ThinLock helper class that handles thin lock case conditions and transitions to AwareLock

[kouvel]

Refactoring/cleanup such as this is not likely to happen until there is significant interest in changing something of impact in the same area into which such changes could be incorporated