Bump version to 0.90.2-SNAPSHOT
Improve readability and completeness of PrintModelProcessor output

The previous implementation of PrintModelProcessor didn't output all registered
resources from the Jersey ResourceModel. The output is now complete, sorted and
printed through the logging subsystem instead of being dumped to stdout.
Update to syslog4j 0.9.50 (because hooray for locales!)
Ensure that searchHits is not null in AlertConditions

Under some conditions FieldValueAlertCondition#getSearchHits and MessageCountAlertCondition#getSearchHits
could return null which caused problems further downstream, when consumers of the respective AlertCondition
expected to always get a list of messages (though being empty if there weren't any search hits).

Refs Graylog2/graylog2-alarmcallback-slack#1
Harden disk-based message cache and implement size limit

The implementation of the disk-based message cache using MapDB now supports additional
settings for various parts of the message spool:

* `message_cache_max_size` for specifying the maximum size of the on-disk spool files
(default: off)
* `message_cache_compaction_interval` for specifying the interval in which the on-disk
spool files should be compacted (default: off)
* `message_cache_enable_compression` for enabling a lightweight compression algorithm
when writing to the on-disk spool files (default: off)

Closes #731
Upgrade to JadConfig 0.8.0