Filesystem backend: Add optional file lock for multiprocess usage

[JWCook]

Currently, the filesystem backend uses a simple threading lock for thread safety. To be safe for multiple processes, it will need a file-level locking mechanism, like py-filelock. That could be added as an optional dependency, and enabled with a keyword argument.

Please comment if this is something you have a use case for!

[rgammans]

I've used it with (a monkey patch to use) multiprocessing.RLock() as my use case would normally be ok with threading but if I cache I'm CPU bound so I need multiprocessing so I can avoid GIL throttling

[JWCook]

Note that multiprocessing.RLock will work only if you have child processes spawned from a common parent process; for concurrent access from multiple unrelated processes, a file-level lock is required.

[rgammans]

Note that multiprocessing.RLock will work only if you have child processes spawned from a common parent process; for concurrent access from multiple unrelated processes, a file-level lock is required.

True, but that was what I was doing,