Shut down the DNS threadpool (#11190)

The DNS threadpool must be explicitly stopped, otherwise Synapse will
hang indefinitely when asked to shut down.

changelog.d/11190.bugfix

@@ -0,0 +1 @@
+Fix a performance regression introduced in 1.44.0 which could cause client requests to time out when making large numbers of outbound requests.

synapse/app/_base.py

@@ -346,6 +346,7 @@ async def start(hs: "HomeServer"):
     # numbers of DNS requests don't starve out other users of the threadpool.
     resolver_threadpool = ThreadPool(name="gai_resolver")
     resolver_threadpool.start()
+    reactor.addSystemEventTrigger("during", "shutdown", resolver_threadpool.stop)
     reactor.installNameResolver(
         GAIResolver(reactor, getThreadPool=lambda: resolver_threadpool)
     )