Improved beast::http::message:
* Add headers::erase
* Set http::message version with std::pair
* Use std::pair for headers::value_type
Add missing includes to make headers compile separately
Universal Port (RIPD-160):
This changes the behavior and configuration specification of the listening
ports that rippled uses to accept incoming connections for the supported
protocols: peer (Peer Protocol), http (JSON-RPC over HTTP), https (JSON-RPC)
over HTTPS, ws (Websockets Clients), and wss (Secure Websockets Clients).
Each listening port is now capable of handshaking in multiple protocols
specified in the configuration file (subject to some restrictions). Each
port can be configured to provide its own SSL certificate, or to use a
self-signed certificate. Ports can be configured to share settings, this
allows multiple ports to use the same certificate or values. The list of
ports is dynamic, administrators can open as few or as many ports as they
like. Authentication settings such as user/password or admin user/admin
password (for administrative commands on RPC or Websockets interfaces) can
also be specified per-port.

As the configuration file has changed significantly, administrators will
need to update their ripple.cfg files and carefully review the documentation
and new settings.

Changes:

* rippled-example.cfg updated with documentation and new example settings:
  All obsolete websocket, rpc, and peer configuration sections have been
  removed, the documentation updated, and a new documented set of example
  settings added.

* HTTP::Writer abstraction for sending HTTP server requests and responses
* HTTP::Handler handler improvements to support Universal Port
* HTTP::Handler handler supports legacy Peer protocol handshakes
* HTTP::Port uses shared_ptr<boost::asio::ssl::context>
* HTTP::PeerImp and Overlay use ssl_bundle to support Universal Port
* New JsonWriter to stream message and body through HTTP server
* ServerHandler refactored to support Universal Port and legacy peers
* ServerHandler Setup struct updated for Universal Port
* Refactor some PeerFinder members
* WSDoor and Websocket code stores and uses the HTTP::Port configuration
* Websocket autotls class receives the current secure/plain SSL setting
* Remove PeerDoor and obsolete Overlay peer accept code
* Remove obsolete RPCDoor and synchronous RPC handling code
* Remove other obsolete classes, types, and files
* Command line tool uses ServerHandler Setup for port and authorization info
* Fix handling of admin_user, admin_password in administrative commands
* Fix adminRole to check credentials for Universal Port
* Updated Overlay README.md

* Overlay sends IP:port redirects on HTTP Upgrade peer connection requests:
  Incoming peers who handshake using the HTTP Upgrade mechanism don't get
  a slot, and always get HTTP Status 503 redirect containing a JSON
  content-body with a set of alternate IP and port addresses to try, learned
  from PeerFinder. A future commit related to the Hub and Spoke feature will
  change the response to grant the peer a slot when there are peer slots
  available.

* HTTP responses to outgoing Peer connect requests parse redirect IP:ports:
  When the [overlay] configuration section (which is experimental) has
  http_handshake = 1, HTTP redirect responses will have the JSON content-body
  parsed to obtain the redirect IP:port addresses.

* Use a single io_service for HTTP::Server and Overlay:
  This is necessary to allow HTTP::Server to pass sockets to and from Overlay
  and eventually Websockets. Unfortunately Websockets is not so easily changed
  to use an externally provided io_service. This will be addressed in a future
  commit, and is one step necessary ease the restriction on ports configured
  to offer Websocket protocols in the .cfg file.
Update `npm test` rippled.cfg to use [server]:
The test now generates a configuration file with the new
configuration sections define by the Universal Port feature.
Remove unused, confusing isXRP(Issue) function.

* Not used in codebase.
* Whether an Issue "is XRP" isn't well-defined.