http: Force-exit event loop after predefined time

This makes sure that the event loop eventually terminates, even if an
event (like an open timeout, or a hanging connection) happens to be
holding it up.

Zcash: cherry-picked from commit ec908d5

src/httpserver.cpp

@@ -461,6 +461,13 @@ void InterruptHTTPServer()
         // Reject requests on current connections
         evhttp_set_gencb(eventHTTP, http_reject_request_cb, NULL);
     }
+    if (eventBase) {
+        // Force-exit event loop after predefined time
+        struct timeval tv;
+        tv.tv_sec = 10;
+        tv.tv_usec = 0;
+        event_base_loopexit(eventBase, &tv);
+    }
     if (workQueue)
         workQueue->Interrupt();
 }