tough: Add locking around datastore write operations

[iliana]

Issue #, if available: #117 (kinda)

Description of changes:
We don't want separate threads using tough to end up writing to its datastore and reading at the same time. #117 suggested making those operations take &mut self, but that bubbles up to requiring most methods on Repository to also take &mut self.

This adds a RwLock to Datastore, allowing "any number of readers to acquire the lock as long as a writer is not holding the lock".

The difference between this and &mut self is that this is ensured at runtime, which may cause a deadlock (I'm not aware of a way this is possible, because the guard drops out of scope before a reader is even returned), as opposed to ensuring against a data race at compile time, which may just cause borrowck to yell politely at you.

I've tested that this does not, in fact, cause a deadlock in updog.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[jahkeup]

I re-read the description, so I see the sorta-ack on my question. Ship it! 🚢

[sam-aws]

✔️