... to support multiple HttpHeadersProviders

fixes #647

closes #560

The info should be updated when the registration has changed. In order
to prevent a stale instance info, the sba client includes a "startup"
timestamp in the metadata as default. So when the instance is restarted
the registration changes and the info is updated, even when the instance
isn't considered as down in between.

To prevent this when using a discovery client you need to add a value
to the metadata which changes with every restart.
(e.g. `random=${random.value}`)

fixes #556

Don't output metadata in Registration.toString()

It's indended by the Jackson developers to extend StdDeserializer and
not the JsonDeserializer

... to prevent registration of the controller from standard spring
mechanisms

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

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.