- Avoid generating piles of temporary strings simply to add a newline to
  things.
- force body to a string (was an array, join with "") to make FTW send a
fixed-size request (avoid chunking).

The idea here is to avoid the previous behavior that logstash sends 2
chunks per event to index (once for bulk header, once for document).
Many small chunks isn't very efficient, and until we make a more
efficient chunk sender, this will suffice.
- upgrade ftw. This new version sets TCP_NODELAY on all connections.
Because of this change, the elasticsearch_http output can send data at a
much faster rate (since we aren't waiting for a full segment before
publishing data)

Before this patch, locally I measured elasticsearch_http at 1500 eps;
After this patch, I measured elasticsearch_http at 8500 eps.