Use join instead of run, to avoid infinite rendering errors

[edkeeble]

When many instances of the async-image component are present simultaneously (our example is an infinite scrolling page of photos), we encountered an "infinite rendering invalidation detected" error. It appears to be a similar issue to the one described here: ember-redux/ember-redux#69, wherein the use of run creates a new run loop for each async-image instance.

In order to avoid this issue, and as a general performance improvement, I've switched to using join, so that each async-image's _onload or _onError will queue itself on an existing run loop, if available.