Add configuration/staus binding/subscription method

[myme]

xconfiguration and xstatus trigger feedback on changes, but they are also non-transient and can be fetched at any time. A very nice convenience method would be to allow binding a callback to configs and statuses which is invoked with the initial value, as well as being called back on changes.

Possible names are .bind() or .subscribe(). "Subscribe" might be a better fit, as "bind" implies more of an end-to-end data binding between elements.

Requirements:

• Feedback APIs must be changed to return a Promise which resolves on successful registration.
• Add a "Subscribable" interface which is mixed into Config and Status.
• Subscribable has a .subscribe() method which first registers feedback, then fetches the value.
• To avoid races, the feedback registration promise must be resolved before the callback is invoked with the current value, from then on, the callback is invoked on all feedback.
• Feedback which arrives before the fetched value is returned, should be queued to avoid data races.