Setup a periodic filter flush thread.

This got lost in 9542246 and unbreaks LOGSTASH-1284.
- disable flush in multiline filter since the codec is preferred now and
the semantics of flushing have changed (#689)
Merge pull request #689 from bernd/LOGSTASH-1284

Setup a periodic filter flush thread. (LOGSTASH-1284)
Merge pull request #762 from logstash/disable-flush-in-multiline-filter

- disable flush in multiline filter since the codec is preferred now and
Merge pull request #763 from untergeek/master

Add GeoJSON capability