http: use current Tornado IOLoop when stopping. (Fixes #1715).

This is in response to the breaking change in Tornado v5.0 where
IOLoop.instance is now a deprecated alias for IOLoop.current.
More info at https://www.tornadoweb.org/en/stable/releases/v5.0.0.html

docs/changelog.rst

@@ -5,6 +5,13 @@ Changelog
 This changelog is used to track all major changes to Mopidy.
 
 
+v2.2.2 (UNRELEASED)
+===================
+
+- HTTP: Fix hang on exit due to change in Tornado v5.0 IOLoop. (Fixes:
+  :issue:`1715`, PR: :issue:`1716`)
+
+
 v2.2.1 (2018-10-15)
 ===================
 

mopidy/http/actor.py

@@ -100,20 +100,21 @@ class HttpServer(threading.Thread):
 
         self.app = None
         self.server = None
+        self.io_loop = None
 
     def run(self):
         self.app = tornado.web.Application(self._get_request_handlers())
         self.server = tornado.httpserver.HTTPServer(self.app)
         self.server.add_sockets(self.sockets)
 
-        tornado.ioloop.IOLoop.instance().start()
+        self.io_loop = tornado.ioloop.IOLoop.current()
+        self.io_loop.start()
 
         logger.debug('Stopped HTTP server')
 
     def stop(self):
         logger.debug('Stopping HTTP server')
-        tornado.ioloop.IOLoop.instance().add_callback(
-            tornado.ioloop.IOLoop.instance().stop)
+        self.io_loop.add_callback(self.io_loop.stop)
 
     def _get_request_handlers(self):
         request_handlers = []