saver needs to use file based mutex locking

[JonJagger]

I would like to increase the puma workers to eg 4 or 8
This will make the integration tests run much faster.
I can't do this at present because the per-kata-id mutexes are process based and not file based.
So:

• switch to file-based mutex locking
• bump the puma workers
• confirm integration tests run faster