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

fixes #590 (new client - old server)

fixes #578

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

fixes #515.

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

https://github.com/spring-cloud/spring-cloud-netflix/pull/1372 has been
merged. So it is save to remove the local workaround.