long-running processes could emit "progress" events

[pinheadmz]

Some RPC calls like wallet rescan return success: true right away even though from a UX perspective, the rescan has just begun:

hsd/lib/wallet/http.js

Lines 189 to 201 in eea15c2

	this.post('/rescan', async (req, res) => {
	if (!req.admin) {
	res.json(403);
	return;
	}
	const valid = Validator.fromRequest(req);
	const height = valid.u32('height');
	res.json(200, { success: true });
	await this.wdb.rescan(height);
	});

Other RPC calls like the WIP dumpzone or pruneblockchain may timeout long before completion since the http timeout is only 5 seconds (from brq).

I was thinking we could establish a convention where these calls always return some kind of "in progress" message once initiated so they don't time out, and then the process itself can emit "progress" events that can either be listened to by the caller (i.e. bob wallet UI) or even from the command line similar to hsw-cli listen