Bump version to 0.93.0-SNAPSHOT
Disable confusing DEBUG log messages regarding Lucene Expressions

Using Lucene Expressions for the dynamic scripting Feature of Elasticsearch are
completely optional but org.elasticsearch.script.ScriptModule will complain about
being unable to load org.apache.lucene.expressions.Expression on DEBUG level
nonetheless. By setting the log level of org.elasticsearch.script to WARN this
message is simply not printed.
Revert "Harden disk-based message cache and implement size limit"

Unfortunately setting a size limit for MapDB would lead to a non-functional
message cache if the limit was hit. Additionally, online compaction (e. g.
while the server was running and still processing messages) didn't work at
all so that the system couldn't recover from a once filled cache at all.

This reverts commit 3404896511f3fb437b879daa62501636f3a5d532, refs #731.
Change wording from Configuration bundles to Content packs in Swagger

TODO: Change wording in the code as well to prevent further confusion in the future.
Unbreak message cache deserializing. Related to #755.

The deserializer did not add additional fields back to the message.
Added a test for that as well.
Inject the ObjectMapper and copy it to allow customization.
Add method to DateTools to get user timezone id
Fix repeated registration of custom SimpleModule in ServerObjectMapperProvider
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
Remove unused MetricsHost class
Do not add timestamp to assembly output when using release profile.
Remove obsolete build scripts. Use maven assembly.
Move checkstyle.xml into config/ directory.
Add @Nullable to alert condition ids, fixes #762

When an alert condition is created the given id is null, as mongodb will
generate one for it.
Stop the server process on dependency injection errors.

Otherwise the process would just hang and do nothing.
Update Version class in REST client
Use correct route to delete index. Fixes #960.
Increase maximum supported ES version to 1.4.99