Allow configuring multiple concurrent Elasticsearch requests via the worker parameter

[cmacknz]

Add support for the worker configuration parameter to the shipper Elasticsearch output. Each additional worker should increase the number of concurrent requests to Elasticsearch that can be in flight.

We should strive to make the behaviour the same as the existing Beats' Elasticsearch output (https://www.elastic.co/guide/en/beats/filebeat/current/elasticsearch-output.html#_worker). Requests should be distributed to each worker randomly, as is currently done in the existing Beats outputs.

Acceptance Criteria:

• A test exists proving that increasing the worker configuration increases the number of concurrent requests to Elasticsearch.

[cmacknz]

Closing as duplicate of #149