Refactor PlatformMonitor into PlatformMutex and PlatformMonitor

As suggested by Per Liden when PlatformMonitor was introduced by JDK-8210832 we should refactor PlatformMonitor into a simpler PlatformMutex extended by PlatformMonitor. That would potentially allow other synchronization objects e.g. Zlocks, to be replaced by a suitable PlatformX class.

As Per noted we need to look at the construction/destruction issues surrounding such usages. In general deleting a synchronization object has to be very carefully managed.