Improve shutdown process

Zcash: cherry-picked from commit 793667af1c31835e0eefcdd283930bb89cfeda8f

src/httpserver.cpp

@@ -479,6 +479,8 @@ void StopHTTPServer()
     }
     if (eventBase) {
         LogPrint("http", "Waiting for HTTP event thread to exit\n");
+        // Exit the event loop as soon as there are no active events.
+        event_base_loopexit(eventBase, nullptr);
         // Give event loop a few seconds to exit (to send back last RPC responses), then break it
         // Before this was solved with event_base_loopexit, but that didn't work as expected in
         // at least libevent 2.0.21 and always introduced a delay. In libevent