Generalize the ephemeral build managers so that any manager may manage a builder spawned by any other manager.

data/queue.py

@@ -78,7 +78,8 @@ class WorkQueue(object):
   def get(self, processing_time=300):
     """
     Get an available item and mark it as unavailable for the default of five
-    minutes.
+    minutes. The result of this method must always be composed of simple
+    python objects which are JSON serializable for network portability reasons.
     """
     now = datetime.utcnow()