This makes sure that the headers are written to the output stream
before the async dispatch. Otherwise we end up with strange NPEs
when flushing the buffer on writing the body.

fixes #715

..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