Incorporate feedback and FLUME-1782/FLUME-1972.

Rename ElasticSearchIndexRequestBuilderer->ElasticSearchIndexRequestBuilderFactory.
Extract AbstractElasticSearchIndexRequestBuilderFactory.
Move date-formatting/current-time-check from ElasticSearchSink->AbstractElasticSearchIndexRequestBuilderFactory.
Use JodaTime for current-time-check.
Add missing ASF licence headers.