[FEATURE ember-eager-update-url] Eagerly update link-to urls

Previously, the url in the address bar would only update
at the very end up the transition. This change causes the
url update to happen immediately unless the transition
was aborted/redirected within the same run loop, which
provides for a better UX 99% of the time.
Merge pull request #4122 from machty/eager-url-update

[FEATURE ember-eager-update-url] Eagerly update link-to urls