clean up old BeaconStates.

[rolfyone]

We should probably provide at least a debug tool to cleanup the states table.

Currently if you switch from archive -> prune mode, we'll not load the archive tables so they're not accessible, but the data remains. Generally speaking, its likely that people are switching from archive mode to cleanup space, so it'd be nice to be able to at the very least clean this as a debug tool...

An alternative here would be to verify the table is empty when we start in prune or minimal mode and slowly clean it up if its not empty (oldest first), but this would be more complicated.... Maybe we should discuss the best option here...