[rabbitmq] March Hare 2.1.0+

[Release notes](http://blog.rubyrabbitmq.info/blog/2014/01/16/march-hare-2-dot-1-0-is-released/): nothing that may affect Logstash, though.
kv=cleanup
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...
Merge pull request #979 from kurtado/doc/filter/kv

kv=cleanup (docs improvements)
Merge pull request #970 from michaelklishin/patch-1

[rabbitmq] March Hare 2.1.0+
Merge git://github.com/logstash/logstash