What to do with data and keys from no-longer-configured onion services?

We should define some mechanism to decide how long to keep persistent state and keys for onion services that are not configured or not running.

(For deleting expired keys from running onion services, see #1198 (closed))

If we were to delete them immediately, it wouldn't be possible to turn an onion service off and on again, which might surprise some users. But if we were were to keep them forever, that would present a surprising behavior, since users might not expect to retain information about expired onion keys forever.