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/module/app/main/Application.cpp

@@ -801,6 +801,10 @@ public:
         //
         // Allow RPC connections.
         //
+#if RIPPLE_ASYNC_RPC_HANDLER
+        m_rpcHTTPServer->setup (m_journal);
+
+#else
         if (! getConfig ().getRpcIP().empty () && getConfig ().getRpcPort() != 0)
         {
             try
@@ -820,6 +824,7 @@ public:
         {
             m_journal.info << "RPC interface: disabled";
         }
+#endif
 
         //
         // Begin connecting to network.