[Maps] use grid-agg layer as default layer when uploading large geojson point files

[thomasneirynck]

When uploading a featurecollection with more than 10000 points, consider use a grid-agg source instead of a vector layer.

10000 docs is the limit of elasticsearch documents. for points, the grid-agg can roll these up.

[elasticmachine]

Pinging @elastic/kibana-gis