Issue #200: change from temp to permanent tables

The temporary tables used in database version 2 would disappear when the
database was sometimes closed when the app was put in the background.
Switch to permanent tables to avoid this issue. Now we need a way to
occasionally clean up though to avoid endless expansion.
Issue #200: delete session after resetting loaders

Hook into onLoaderReset to delete session data once the fragments are
destroyed, so that our database doesn't keep growing without bound.
Issue #200: add one time clean of the database

Add a hook to remove residue in the database if the application was
terminated abruptly.
Fix #200: move SessionProvider into ThingProvider

The code in SessionProvider is getting specific to ThingProvider with
the one time cleaning code calling out specific tables. So move the
SessionProvider code into ThingProvider. SubredditProvider didn't need
to extend SessionProvider so make it just extend BaseProvider.