Universal Port: Simplified administration for rippled servers

This implements part of the "Universal Port" feature. It allows peer, RPC over http, RPC over HTTPS, Websocket, and Secure Websockets to be served on a single listening port. In this version, websockets is disabled and only legacy peer connections are allowed to become peers. Peer HTTP upgrade requests are given a "503 Service Unavailable" with a JSON content body with a handful of IP addresses to visit.
Reviewers: @ximinez, @JoelKatz 
In order to hand off a Websockets or Secure Websockets connection from the `HTTP::Door` it needs to be possible to construct a websocket connection from an already-existing `boost::asio::ip::tcp::socket` or `boost::asio::ssl::stream` and with some bytes that have already been read on the connection. Furthermore, the websocket library needs to use a provided `boost::asio::io_service` instead of bringing its own. Making these changes to websocketpp is difficult.