Fix race condition between starting HTTP server thread and setting EventBase()

Split StartHTTPServer into InitHTTPServer and StartHTTPServer to give
clients a window to register their handlers without race conditions.

Thanks @ajweiss for figuring this out.

src/httpserver.h

@@ -20,7 +20,14 @@ struct event_base;
 class CService;
 class HTTPRequest;
 
-/** Start HTTP server */
+/** Initialize HTTP server.
+ * Call this before RegisterHTTPHandler or EventBase().
+ */
+bool InitHTTPServer();
+/** Start HTTP server.
+ * This is separate from InitHTTPServer to give users race-condition-free time
+ * to register their handlers between InitHTTPServer and StartHTTPServer.
+ */
 bool StartHTTPServer(boost::thread_group& threadGroup);
 /** Interrupt HTTP server threads */
 void InterruptHTTPServer();