Fixing NPE if backlog is configured to be zero.

Fixes #865
Start tests for ServerStatus class.
Fix ServerStatus test wrt event bus.
Make pausedLatch in KafkaInput volatile to avoid visibility problems.
Add awaitRunning() method to ServerStatus.
Move ServerStatus class to graylog2-plugin-interfaces.

This is needed to be able to access the server status from input
plugins.

Requires BaseConfiguration and ProcessingPauseLockedException to move as
well.
Merge pull request #674 from Graylog2/move-server-status-to-plugins

Move ServerStatus class to graylog2-plugin-interfaces
Fix KafkaInput startup dead-lock.

The input was paused until the event bus sends a RUNNING lifecycle event
but the input was only subscribed to the event bus after the RUNNING
event already happened.
Upgrade to Jersey 2.11 and HK2 2.3.0-b10
Get rid of reflection-based monkey-patching of HK2

Instead of trying to override hard-coded defaults in the Injections class,
now the standard Java SPI (ServiceLoader) is being used. The required
Injector instance is created and held by the new GuiceInjectorHolder class
and used by Graylog2ServiceLocatorGenerator.