Take lock over Snapshotter state during callback

[eddyashton]

Another fix spun out of #6616.

This Snapshotter class has a std::mutex member, which is locked in most of its member functions to prevent concurrent access to its member state. The one function that doesn't try to lock, is snapshot_, the callback that comes via the ThreadMessaging system so is almost guaranteed to race with other calls. I think the reason we didn't lock there is that locking for the whole function causes a deadlock (this callback tries to .commit() a Tx, which calls back into the Snapshotter to work out if that should trigger another snapshot...). But that's fine, we can drop the lock while we're not accessing any member state.

This isn't great, and potentially risks leaving orphaned items in the pending_snapshots collection? But I think the fix is worthwhile for now, until we have a better task system to work with.