Use native http2 instead of spdy

[matz3]

Expected Behavior

Native HTTP/2 protocol from Node.js is used.

Current Behavior

SPDY protocol is used for HTTP/2 server.

Details

This currently depends on the native HTTP/2 of express as it's used as the underlaying framework for our server: expressjs/express#3730

However we might also switch to a different framework such as fastify, which supports native HTTP/2 and out of the box provides an asynchronous middleware engine compatible with Express and Restify middlewares (https://github.com/fastify/fastify/blob/master/docs/Middlewares.md) so we wouldn't need to re-write our existing middlewares.