Cache tasks by uuid in PlayIterator for O(1) lookups

Rather than repeatedly searching for tasks by uuid via iterating over
all known blocks, cache the tasks when they are added to the PlayIterator
so the lookup becomes a simple key check in a dict.

test/units/executor/test_play_iterator.py

@@ -329,7 +329,7 @@ class TestPlayIterator(unittest.TestCase):
         # test the high-level add_tasks() method
         s = HostState(blocks=[0,1,2])
         itr._insert_tasks_into_state = MagicMock(return_value=s)
-        itr.add_tasks(hosts[0], [3,4,5])
+        itr.add_tasks(hosts[0], [MagicMock(), MagicMock(), MagicMock()])
         self.assertEqual(itr._host_states[hosts[0].name], s)
 
         # now actually test the lower-level method that does the work