Restore the fitler thread safety check that was mysteriously lost, I think, in the new pipeline rewrite for 1.2.0 (LOGSTASH-1828)

Also mark the metrics filter as not threadsafe.
Include filter-worker validation before saying the config is good.
Merge pull request #981 from jordansissel/abort-on-not-threadsafe-filters

Restore the fitler thread safety check that was mysteriously lost, I thi...