fix `too late` transition error

[ghost]

when you click around very fast, a console error happens - need to look into this

[taras]

I put some effort into trying to figure this out. It's not a trivial problem and it's not clear how to solve it. This problem occurs when you trigger a transition while there is one in progress. It also, seems to only happen when you're in the later stage of the transition.

I changed the demos to throttle data change to only cause data change after transition is finished. I explicitly specified throttle delay to match transition duration + 20ms. This seams to work reliably enough. It's an ok solution for now but we need a better solution long term.

It would be excellent if we could use an ember-concurrency task with a transition. This wold allow us to programmatically cancel and restart transitions. What this looks like exactly, I'm not sure. It'd be a fun experiment for someone who's interested.