move client/server from HTTP to WebSocket

[joshsh]

WebSocket has a number of benefits over HTTP in SmSn:

• WebSocket potentially cuts down on latency in commands and responses by providing long-lived connections. This is particularly important when the server is hosted on a remote (e.g. cloud) machine.
• it enables the server to notify the client without explicit polling
• WebSocket the default protocol for communication with Gremlin Server. A greater variety of TinkerPop-based tools support it. Concurrent connections are possible from Brain-mode and Gremlin Server.

There will be no good reason to continue supporting REST once we have a solid WebSocket solution.

[joshsh]

This is done. WebSocket and also HTTP are supported; it was simple enough to maintain support for both.