Ensure that "track" event is only fired for "send" direction m-sections.

[taylor-b]

Fixes #1013

Also does a bit of refactoring. The steps that used to be listed in the
"dispatch a receiver" section are now covered by "create a receiver" and
"stop a transceiver".

[aboba]

Is most of the text in lines 4927 - 4967 and 4987-5022 contained elsewhere so it is redundant? Trying to understand if all the steps described in that text are still covered once this material is deleted.

[taylor-b]

@aboba That's what it seemed to me.

The "dispatch a receiver" steps for creating a track were identical to those in "create a receiver".

And the steps for "when a track is removed" are covered by the transceiver being stopped. This is the only time when a track should go to "ended", since that's a terminal state, and stopping a transceiver is the only terminal thing for its RtpReceiver. The differences I see are:

• The previous text mentioned that "an event" is fired when the track state goes to "ended". It looks like this is missing from the "stop a transceiver" steps; I'll add it.
• A task is queued to remove the receiver from the "set of receivers". This is no longer relevant here; there's a "set of transceivers" instead.