`flush` method for BulkProcessor class.

This is for #5570.

There is no explicit method `flush/execute` in [BulkProcessor][1] class. This can be useful in certain scenarios. Currently it requires to close and create a new BulkProcessor if one wants an immediate flush.

[1]: http://javadoc.kyubu.de/elasticsearch/v0.90.0/org/elasticsearch/action/bulk/BulkProcessor.html