Hi Keith,

Can you please review these changes to add locking for snapshot operations.

In particular, there is an API change in here so drop operations only have a single "snapshot" config key, with an optional structure argument to specify ranges (all, from or to). The example code should make this clear: let me know if you don't like it, but I think it's cleaner to have a single config key indicating that we're dropping snapshots.

The rest of the changes relate to locking snapshots, tracking the locks and releasing them when finishing tracking. It's passing tests.

Thanks,
Michael.