Create more or faster workers

[matthew-white]

At the moment, run-server.js spins up just one worker (I think). However, we want run-server.js to spin up multiple workers. (We've talked about 4 being a good number.)

On most machines, start-odk.sh in the central repository will use pm2 to spin up multiple instances. However, it seems like it'd be good to have more than one worker per instance.

[matthew-white]

Right now, I think a Backend worker can wait up to 25 seconds to check for work to do:

central-backend/lib/worker/worker.js

Lines 81 to 98 in c90025b

	// main loop. kicks off a check and attempts to process the result of the check.
	// if there was something to do, takes a break while that happens; the runner will
	// call back into the scheduler when it's done.
	// if there was nothing to do, immediately schedules a subsequent check at a capped
	// exponential backoff rate.
	const worker = (container, jobMap = defaultJobMap, defaultDelay = 3000) => {
	let enable = true; // we allow the caller to abort for testing.
	const check = checker(container);
	const run = runner(container, jobMap);
	const status = new Status();
	const withStatus = (x, chain) => { status.set(x); return chain; };
	const report = reporter(container.Sentry);
	// this is the main loop, which should already try to hermetically catch its own
	// failures and restart itself.
	const now = (delay = defaultDelay) => {
	if (!enable) return;
	const wait = () => { waitFor(min(delay * 2, 25000)); }; // eslint-disable-line no-use-before-define

25 seconds seems like a fair amount of time to wait. Though it's also not common for a user to be actively waiting for an event to be processed: most of the time, async work isn't needed immediately. But should we decrease 25 to a smaller number? Maybe that wouldn't be needed if we added more workers, as that would decrease the average wait time.