Unbreak #to_epoch method in datadog output plugin. (LOGSTASH-1427)

The event timestamp might already be a Time object.
- allow using asynchronous replication setting in index requests
Merge pull request #700 from logstash/es-http-async

- allow using asynchronous replication setting in index requests
Merge pull request #690 from bernd/LOGSTASH-1427

Unbreak #to_epoch method in datadog output plugin. (LOGSTASH-1427)
Instead of adding a bool for async_replication in es_http, allow multiple flags.
Merge pull request #701 from logstash/es-http-async

elasticsearch_http replication type
Merge git://github.com/logstash/logstash