Employ asyncio locks for pipes to avoid drain errors in aiohttp

https://github.com/aio-libs/aiohttp/issues/2934#issuecomment-401304213

pypubsub.py

@@ -296,6 +296,7 @@ class Subscriber:
         self.connection = connection
         self.acl = {}
         self.server = server
+        self.lock = asyncio.Lock()
 
         # Set topics subscribed to
         self.topics = [x for x in request.path.split('/') if x]
@@ -351,7 +352,8 @@ class Subscriber:
         js = b"%s\n" % json.dumps({"stillalive": time.time()}).encode('utf-8')
         if self.old_school:
             js += b"\0"
-        await self.connection.write(js)
+        async with self.lock:
+            await self.connection.write(js)
 
 
 class Payload:
@@ -391,9 +393,11 @@ class Payload:
             if all(el in self.topics for el in sub.topics):
                 try:
                     if sub.old_school:
-                        await sub.connection.write(ojs)
+                        async with sub.lock:
+                            await sub.connection.write(ojs)
                     else:
-                        await sub.connection.write(js)
+                        async with sub.lock:
+                            await sub.connection.write(js)
                 except Exception:
                     bad_subs.append(sub)
         return bad_subs