Add the ability to recover `wallet.db` in case the password was pruned from the keystore

[hsjoberg]

It's possible to recover from this situation by deleting wallet.db and then do initWallet again with the old seed. channel.db isn't actually encrypted by the password so it will just work.

We need a procedure that:

1. Closes down lnd
2. Removes wallet.db
3. Starts up lnd
4. Calls initWallet with the original seed words.