Add file-based locking to cache files

[fluffy-critter]

To better support concurrency, add a Cache.lock primitive which would look something like

with cache.lock(prefix, url) as lock:
    previous = lock.get(schema_version)
    # ...
    lock.save(current)

While a lock is held on a file, any other attempt at acquiring that lock should block.

[fluffy-critter]

At this point you might wonder why I don’t just move to using SQLite for the backing store. The thing is,