[Maps] Use composite aggregation instead of terms aggregation for join

[nreese]

Currently, joining a vector layer in the GIS app uses a terms aggregation. This should be updated to use a composite aggregation for better scaling and less cluster stress.

[elasticmachine]

Pinging @elastic/kibana-gis

[thomasneirynck]

Let's close. This asks for a specific way of implementing a large data-fetch. We can re-open when we have specific requests to support choropleth-mapping beyond the 65k limit.