issue #615: ensure 4GB max_message_size is configured for task workers.

This 4GB limit was already set for MuxProcess and inherited by all
descendents including the context running on the target host, but it was
not applied to the WorkerProcess router.

That explains why the error from the ticket is being raised by the
router within the WorkerProcess rather than the router on the original
target.

ansible_mitogen/process.py

@@ -72,6 +72,8 @@ ANSIBLE_PKG_OVERRIDE = (
     u"__author__ = %r\n"
 )
 
+MAX_MESSAGE_SIZE = 4096 * 1048576
+
 worker_model_msg = (
     'Mitogen connection types may only be instantiated when one of the '
     '"mitogen_*" or "operon_*" strategies are active.'
@@ -502,6 +504,7 @@ class ClassicWorkerModel(WorkerModel):
             # with_items loops.
             raise ansible.errors.AnsibleError(shutting_down_msg % (e,))
 
+        self.router.max_message_size = MAX_MESSAGE_SIZE
         self.listener_path = path
 
     def _on_process_exit(self):
@@ -692,7 +695,7 @@ class MuxProcess(object):
         self.broker = mitogen.master.Broker(install_watcher=False)
         self.router = mitogen.master.Router(
             broker=self.broker,
-            max_message_size=4096 * 1048576,
+            max_message_size=MAX_MESSAGE_SIZE,
         )
         _setup_responder(self.router.responder)
         mitogen.core.listen(self.broker, 'shutdown', self._on_broker_shutdown)