Add new endpoints to api-server

[thomcom]

Closes https://github.com/rapidsai/node/issues/438

This branch adds two new endpoint families: /particles and /quadtree.

The particles endpoint returns a striped buffer that is used to display coordinates in the particles UI client.

The quadtree endpoint exposes the API endpoints for quadtree creation, setting a polygon for the quadtree, and querying points contained within the polygon.

[thomcom]

This is ready for review.

[thomcom]

@trxcllnt