Introduce fallback provisions & extension points

[Abnaxos]

Many of the library features have two variants: one with a ShutdownFeature and one without. This is a common situation: if a feature is present, use it, otherwise don't. Another example could be JMX: register an MBean if we're using JMX, otherwise don't.

Most such situations could be solved by a no-op fallback provision declared in the feature, e.g.:

@Provision(fallback = true)
protected ShutdownController shutdownController() {
  return ShutdownController.NOOP;
}

The traits of fallback provisions are:

• multiple fallbacks don't conflict, each feature uses its own fallback implementation
• if a matching non-fallback provision is present, it implicitly overrides all fallbacks
• multiple non-fallback provisions conflict, as usual

The same mechanism is usable for implementing optional extension points: provide a fallback noop extension point if none could be found.