simplify acceptor setup logic

After reading run_until_complete doc :)

src/ZEO/asyncio/server.py

@@ -224,10 +224,8 @@ class Acceptor(object):
             cr = loop.create_unix_server(self.factory, addr, ssl=ssl)
 
         f = asyncio.async(cr, loop=loop)
+        server = loop.run_until_complete(f)
 
-        @f.add_done_callback
-        def listenting(f):
-            server = f.result()
         self.server = server
         if isinstance(addr, tuple) and addr[1] == 0:
             addrs = [s.getsockname() for s in server.sockets]
@@ -236,9 +234,8 @@ class Acceptor(object):
                 self.addr = addrs[0]
             else:
                 self.addr = server.sockets[0].getsockname()[:len(addr)]
-        logger.info("listening on %s", str(addr))
 
-        loop.run_until_complete(f)
+        logger.info("listening on %s", str(addr))
 
     def factory(self):
         try: