Improvement: Engine API http and ws on the same port

[yorickdowne]

For easier configuration and to match every other EL out there, it'd be very helpful if http and ws for the Engine API were on the same port 8551 by default, instead of on 8550 and 8551 respectively.

WS is an upgrade from HTTP after the initial connection.

    --engine-api              Enable the Engine API [=false].
     --engine-api-port         Listening port for the Engine API [=8550].
     --engine-api-address      Listening address for the Engine API [=127.0.0.1,
                               meaning local host only].
     --engine-api-ws           Enable the WebSocket Engine API [=false].
     --engine-api-ws-port      Listening port for the WebSocket Engine API
                               [=8551].
     --engine-api-ws-address   Listening address for the WebSocket Engine API
                               [=127.0.0.1, meaning local host only].

[jangko]

TODO:

• devise a common HHTP server than can be used by JSON RPC, Graphql, and websocket services.
• both engine api and ws services need to use common HTTP server instead of it's own HTTP server.
• detect HTTP upgrade and route the request to ws handler.
• scan GET/POST path contains /graphql and route the request to graphql handler.
• request maybe JSON RPC if the ContentType contains application/json, route it to rpc handler.

[jangko]

related issues:

• extract server handler from httpserver.nim nim-graphql#113
• Support for combo HTTP server nim-json-rpc#174
• Is there a way to use this with a different webserver? nim-websock#133