issue #587: 2.8 PlayContext.connection no longer contains connection name

Not clear what the intention is here. Either need to ferret it out of
some other location, or just stop preloading the connection class in the
top-level process.

ansible_mitogen/strategy.py

@@ -40,6 +40,12 @@ import ansible_mitogen.process
 import ansible
 import ansible.executor.process.worker
 
+try:
+    # 2.8+ has a standardized "unset" object.
+    from ansible.utils.sentinel import Sentinel
+except ImportError:
+    Sentinel = None
+
 
 ANSIBLE_VERSION_MIN = '2.3'
 ANSIBLE_VERSION_MAX = '2.8'
@@ -261,14 +267,17 @@ class StrategyMixin(object):
             name=task.action,
             mod_type='',
         )
-        ansible_mitogen.loaders.connection_loader.get(
-            name=play_context.connection,
-            class_only=True,
-        )
         ansible_mitogen.loaders.action_loader.get(
             name=task.action,
             class_only=True,
         )
+        if play_context.connection is not Sentinel:
+            # 2.8 appears to defer computing this value until it's inside the
+            # worker. TODO: figure out where this value has moved.
+            ansible_mitogen.loaders.connection_loader.get(
+                name=play_context.connection,
+                class_only=True,
+            )
 
         return super(StrategyMixin, self)._queue_task(
             host=host,