Re: [ServiceWorker] Event queuing model for SW (#649)

Updated this part: 7a8bc4223462f2e6000b2b2573a009f67c686be4.

- Added two task sources: the _handle fetch task source_ and the _handle functional event task source_.
- A registration owns its own _task queues_ that back up the tasks (having the above task sources) from the active worker's task queues.
- Defined Terminate Service Worker algorithm where back up the tasks to the registration's task queues as describe above.
- Run Service Worker algorithm re-queues the tasks from the registration's task queues to the service worker's event loop's task queues if the service worker is an active worker.

* As discussed in the f2f, tasks for message events are not re-queued as they are sent to specific service workers instead of an active worker.

@annevk @jakearchibald @slightlyoff Could you review it?

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