Run register_service in a separate thread in `backend.http`.

The Zeroconf registration part may randomly get stuck, resulting in the
web server not being properly started.

It's therefore better to run the Zeroconf registration process
asynchronously, for it's not strictly required for the web server to
execute.

platypush/backend/http/__init__.py

@@ -251,6 +251,7 @@ class HttpBackend(Backend):
         self.disable_websocket = disable_websocket
         self.websocket_thread = None
         self._websocket_loop = None
+        self._service_registry_thread = None
         self.bind_address = bind_address
 
         if resource_dirs:
@@ -328,6 +329,10 @@ class HttpBackend(Backend):
             self._websocket_loop.stop()
             self.logger.info('HTTP websocket service terminated')
 
+        if self._service_registry_thread and self._service_registry_thread.is_alive():
+            self._service_registry_thread.join(timeout=5)
+            self._service_registry_thread = None
+
     def _acquire_websocket_lock(self, ws):
         try:
             acquire_ok = self._websocket_lock.acquire(
@@ -453,14 +458,16 @@ class HttpBackend(Backend):
 
         return proc
 
-    def run(self):
-        super().run()
+    def _register_service(self):
         try:
             self.register_service(port=self.port)
         except Exception as e:
             self.logger.warning('Could not register the Zeroconf service')
             self.logger.exception(e)
 
+    def run(self):
+        super().run()
+
         if not self.disable_websocket:
             self.logger.info('Initializing websocket interface')
             self.websocket_thread = threading.Thread(target=self.websocket)
@@ -483,5 +490,8 @@ class HttpBackend(Backend):
                 + 'for the webserver (e.g. nginx)'
             )
 
+        self._service_registry_thread = threading.Thread(target=self._register_service)
+        self._service_registry_thread.start()
+
 
 # vim:sw=4:ts=4:et: