experimental elasticsearch connector

first draft of an elasticsearch connector
known limitations and problems:

• complex column types (geo_points, arrays/maps) are not parsed;
• when no index is specified for a documents type/table, all indices, its mappings and columns are taken in consideration. this is not performant at all;
• the (small) test coverage depends on having a elasticsearch cluster running locally;
• table mappings are evaluated in every query (!)
• ES client is constructed in every query... multiple times

based on the work done in prestodb#3240