Review Request for CALCITE-2528. Support Aggregates in ElasticSearch Adapter

[Andrei Sereda]

Hello,

Can somebody please review the following PR: 822
<https://github.com/apache/calcite/pull/822> (CALCITE-2528
<https://issues.apache.org/jira/browse/CALCITE-2528>) ?

It enhances ElasticSearch adapter with native ES aggregations
<https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations.html>
.

Thank you,
Andrei.