fix: use worker to prevent timer throttling

[myandrienko]

Browsers start throttling timers (intervals, timeouts) in background tabs after a period of inactivity. We use timers to send health checks, and missing health checks cause participant to be disconnected.

An active WebRTC connection on a page usually means the page is never considered inactive by the browser, and therefore timers are not throttled. But it's still possible to be in a call and not have an active connection, e.g. if nobody is publishing neither audio nor video.

To handle situations like this, we move timers to a web worker, which are never throttled. Web worker then posts messages to the page on the specified intervals.

Web workers might not be available (banned by CSP, running on Node, etc), so there's also a fallback mode that uses regular timers on the page side.