Adding tunable for the batched output flush interval and explanation
Make flush interval of batched ES output configurable. Defaults to 1s.

We really need to change Periodicals to have runtime intervals with
millisecond precision.

Conflicts:
	graylog2-server/src/main/java/org/graylog2/Configuration.java
	graylog2-server/src/main/java/org/graylog2/periodical/BatchedElasticSearchOutputFlushThread.java