..to gain more performance and reduce gc pressure

fixes #713

closes #713

closes #697

The system.load.average.1m metric is not available on all platforms.
Therefore we ignore errors from fetching it.

closes #697

Fixes stalls in threaddump view due to not completing responses - only
reactive applications were affected so far.

The TaskScheduler used for periodically registering is removed from the
application context and moved to the ApplicationRegistrationListener.

see #478

If the build version of the instance is present in the metadata it's now
also considered and the version is now a attribute on the instance
itself. So when using non-boot application they can provide it via the
registration metadata.

- use MimeMessage with Content-Type text/hmtl
- use a single template for subject and body
- use a Spring aware TemplateResolver

The WebClient's responses should be always consumed, so that underlying
buffers are released.

Fixes #666