SH-4411 Thread/mutex rework between main and worker thread

Have the ::notifyLoadedMeshes() method doing correct locking
and stall avoidance at the same time.  This method now does
lazy mutex lock acquisition (trylock()) and if it fails on
either, it gives up and comes back later.  Capture the maximum
number of sequential failures and report this at the end of
the run in the log.  (So far, with big mesh regions, I've
only seen 1s and 2s.)  Locking/mutex requirements sorted in
other locations as well.  LLMutex gets trylock() method as
well as new LLMutexTrylock scoped locking class.  Clean up
some documentation, more to do.

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