Until now the thread querying the application endpoint can be blocked
for a very long time when the application is inresponsive. With this
commit two new properties `spring.boot.admin.monitor.read-timeout`
(default: 2s) and `spring.boot.admin.monitor.connect-timeout` (default:
5s) are added.

closes #462

The info is now fetched by the server on status change. This way the
info can be used in the notifications and also improves performance when
listing a huge amount of applications in the ui.

closes #349

With this change it is possible to include credentials in the instance
metadata which will then be used to access the client endpoints using
HTTP Basic authorization.

closes #359