Setup a periodic filter flush thread. (LOGSTASH-1284)

This got lost in 9542246 and unbreaks [LOGSTASH-1284](https://logstash.jira.com/browse/LOGSTASH-1284).

I took the `#flusher` method from the FilterWorker class and removed the `@after_filter` stuff. Not sure if you guys want to solve it differently.

What is the policy for filter plugins? Are they supposed to be thread-safe?