service: avoid taking another lock in the usual case

mitogen/service.py

@@ -547,6 +547,9 @@ class Pool(object):
             invoker.service.on_shutdown()
 
     def get_invoker(self, name, msg):
+        invoker = self._invoker_by_name.get(name)
+        if invoker is None:
+            # Avoid acquiring lock if possible.
             self._lock.acquire()
             try:
                 invoker = self._invoker_by_name.get(name)