NMT has comments that it needs to use ThreadCritical for Chunk accounting, particularly if the Chunk is deleted. Instead of using ThreadCritical, which is another hand written pthread or windows implemented lock, use PlatformMutex. The ChunkPool is used before the current thread is created in many cases, so the ChunkPool lock needs to be a global PlatformMutex, and can't use the runtime Mutex interface that wraps PlatformMonitor with rank checking and other nice deadlock detection.

This is the last use of ThreadCritical, so ThreadCritical can be removed.