Update Elasticsearch Output Plugin to retry bulk

Some actions may fail within the ES client bulk call.
Now, some messages (specifically errors 429 and 503s) will be
retried up to 3 times. If there are still actions that are
unsuccessfully indexed, Stud Buffer will continue its current
behavior (to retry indefinitely).

Stud Buffer will replay all events it first attempted to flush
to Elasticsearch. This means duplicate events may find themselves
in Elasticsearch.

(migrated from https://github.com/elasticsearch/logstash/pull/1997 with a few modifications to make 
tests run once plugin is installed)
I think there is a potential race condition here. `retry_flush` is called both by the flush tread at periodic interval and by the submit method. The statement

```ruby
buffer = @retry_queue.size.times.collect { @retry_queue.pop }
```

grabs the `@retry_queue