Wallet API: long-process commands should emit a success/fail status event when completed

See https://github.com/handshake-org/hsd/pull/594#pullrequestreview-640973832

API commands like `deepclean`, `rescan`, and `backup` may take longer to complete than the http timeout allows. Currently commands like this either just timeout, or they return `{success: true}` immediately, while the process runs in the background.

It would be more helpful if these processes could be monitored somehow either (or both):

A status API that can be polled, like:

```
{
rescan: false,
backup: false,
deepclean: true
}
```

...and/or the wallet can emit socket events when long-processes are completed or fail.