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.

The Spring team discourages the use of the `spring.*` configuration
namespace, hence I use the major update and move all properties. With
this commit all client related properties are moved to `sbadmin.client`.

In order to reduce the noise in the application log, only the first
failure when registering is logged on the WARN level. All further
attempts are logged using the DEBUG level.

closes #490

With this commit two new properties were added to ease the client
configuration when used in servlet containers:
`spring.boot.admin.client.management-base-url` and
`spring.boot.admin.client.service-base-url`. Both properties can be used
to specify protocol, host and port for the urls. The paths will still be
guessed and appended to the base-urls. Additionally the servlet context
path is not read from the properties but from the ServletContext itself.

closes #488

in order to get the ConfigurationProperties correctly recognized we need
to provide some getters for the properties.

fixes #493