Ensure render and layout tasks get destructed before main thread finishes

Pipeline now blocks on `Pipeline.exit()` and waits for confirmation that RenderTask and LayoutTask were successfully destructed (they're created inside a block which ensures they go out of scope before the message is sent.

Other notable changes include:
* The blocking recv in main/servo.rc that waits for a constellation exit confirmation was moved out of the spawn and into the main thread.  `Constellation::start()` spawns a new task, so there's no need to block in that task once the ExitMsg is sent off.

* Application initialization was moved out of `run_compositor` and into `CompositorTask`, so it could be stored with the compositor task and not go out of scope until the program's main block finishes.

I had to use SharedPort<()> objects to work around the fact that pipelines get cloned, even though it feels like pipelines should probably be ref counted instead.  I went down that route for a few hours but got bogged down trying to wrap my head around some region/lifetime issues; I would be happy to work on that part again, but would need some mentoring to help me figure out what I was doing wrong and possibly why.  The current solution doesn't feel too bad though.

Fixes #1097.