Allow the individual build manager types to specify how long the queue should wait before retring a job that fails to schedule.

buildman/manager/basemanager.py

@@ -32,8 +32,9 @@ class BaseManager(object):
 
   @coroutine
   def schedule(self, build_job):
-    """ Schedules a queue item to be built. Returns True if the item was properly scheduled
-        and False if all workers are busy.
+    """ Schedules a queue item to be built. Returns a 2-tuple with (True, None) if the item was
+        properly scheduled and (False, a retry timeout in seconds) if all workers are busy or an
+        error occurs.
     """
     raise NotImplementedError