StreamBindings should always notify on sync

Fixes #9337.

This is one possible solution to the issue. Another solution is to drop registerObserver in attribute bindings, because those code paths don't consume the stream binding, but this PR provides a sufficient stopgap.

In Ember 2.0 we can do better by giving bindings CP/Stream-like semantics, i.e. notify+pull instead of push.

cc @rlivsey