Move metrics-jvm into graylog2-shared POM
Add lots of JVM-related metrics to MetricRegistry singleton

The Metrics library provides a lot of JVM-related metrics out of the box which
only have to be registered with the MetricRegistry.

See https://dropwizard.github.io/metrics/3.1.0/apidocs/com/codahale/metrics/jvm/package-summary.html for details

Fixes #759
Generate system notification if GC takes longer than threshold

Long garbage collection runs are usually an early signs of an unhealthy system.
This changeset generates an URGENT system notification of type "gc_too_long" for
the node which suffered the long GC run.

The user can configure the check interval (`gc_check_interval`, default: 1 minute)
and the threshold (`gc_warning_threshold`, default: 1 second) in graylog2.conf.

Fixes #760