It turns out that imho when talking about application groups (identified
by name) and application instances it's just logical to speak of
instances and applications. So the Application class get's renamed to
Instance. So an Application is now a set of Instances with the same name

To avoid some strange config sorting errors using the
DeferredImportSelector, switch to a real AutoConfiguration activated by
a marker bean

fixes #531

fixes #515.

fixes #529

The actuator endpoints are detected by either querying the index
endpoint for spring boot 2.x applications or by probing a configurable
list of endpoints using a OPTIONS request.

Since there is no Heartbeat or InstanceRegisteredEvent when using the
SimpleDiscoveryClient we need to listen on the ApplicationReadyEvent
and register the configured instances

closes #505

* extract InfoUpdater from StatusUpdater
* rename ApplicationJournal to ClientApplicationEventStore
* ClientApplicationEventStore now implements reactor's Publisher
* Reworked AdminApplicationTest to be more sophisticated.