Allow pindexer to handle schema changes itself

[cronokirby]

Right now, when the pindexer schema changes, one needs to know about this inherently, manually wipe the entire database, and then reindex everything from scratch.

Wiping everything is often unnecessary, especially if a minor change in one schema causes other app views to need to be reindexed, unnecessarily, as well.

Having to manually keep track of whether schema changes happened is also error prone.

Solution

App views should know what schema version they have, and we should record it in the state we keep track of for each app view. App views should also know how to reset themselves, so that when we detect that that version doesn't match what we currently have, the app view can safely delete all of the tables it might have created in the past.