Enable asynchronous handling of HTTP-RPC (RIPD-390)

* Activate async code path
* Tidy up HTTP server code
* Use shared_ptr in HTTP server
* Remove check for unspecified IP
* Remove hairtrigger
* Fix missing HTTP authorization check
* Fix multisocket flags in RPC-HTTP server
* Fix authorization failure when no credentials required
* Addresses RIPD-159, RIPD-161, RIPD-390

src/ripple/common/MultiSocket.h

@@ -96,15 +96,6 @@ public:
     private:
         int m_flags;
     };
- 
-    enum Flags
-    {
-        none = 0,
-        client_ssl = 1,
-        server_ssl = 2,
-        server_ssl_required = 4,
-        server_proxy = 8
-    };
 
     typedef beast::asio::HandshakeDetectLogicPROXY::ProxyInfo ProxyInfo;