refactor to not do too much in ReentrantFileLock.__del__

[mattip]

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Internal refactor of ReentrantFileLock.__del__ to avoid the _store_lock context if not needed. Also make sure the lock is deleted and set to None in the __exit__ call, so that __del__ now will be a no-op on the happy path.

This came up in #2206 when looking at PyPy failures. Since PyPy uses a non-refcount garbage collector, __del__ can be called much later than on CPython, leaving unneeded locks hanging around.

[gaborbernat]