WalletDB should re-emit block events

[pinheadmz]

This is a pretty simple feature that would make GUI applications easier, for example right now Bob has to unhook rescan events from the wallet client and MITM them in order to notify the user: https://github.com/kyokan/bob-wallet/blob/master/app/background/wallet/service.js#L888-L890

WalletDB (well, wallet and txdb actually) already emit events for tx, address, and a few others but not for blocks.

Possibly the easiest approach would just be to re-emit events from the wallet's node client right after its done processing them: https://github.com/handshake-org/hsd/blob/master/lib/wallet/walletdb.js#L132-L154

Don't forget to add tests to your PR!