Clarify which event loop task will be used for track in configuration change .

[beaufortfrancois]

As originally posted by @youennf in #83 (comment), it is not very clear whether each clone track will fire in its own event loop task or in the same event loop task when the User Agent detects a change of configuration in a track's underlying source.

[youennf]

WebKit is currently queueing a task to fire each event.
This is simpler, the small downsides is that, two cloned tracks may have different capabilities for a small amount of time.

[jan-ivar]

It seems common in the platform to queue a task per event fired, with some exceptions, like SRD, which cause a lot of grief to implementers.