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 looked into each the clients. does not look like any state is manipulated that would cause race conditions. In terms of state that is changed from within `submit()` both `Stud::Buffer`'s submit call as well as `retry_flush`'s call can be appending to the queue within `retry_push` at the same time. I do not see that as being an issue, unless I'm mistaken.