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.
* Unit tests exposed a problem where nested blocks did not correctly
hit rescue/always portions of parent blocks
* Cleaned up logic in PlayIterator
* Unfortunately fixing the above exposed a potential problem in the
block integration tests, where a failure in an "always" section may
always lead to a failed state and the termination of execution
beyond that point, so certain parts of the block integration test
were disabled.
Also making PlayContext a child class of the Playbook Base class,
which gives it access to all of the FieldAttribute code to ensure
field values are correctly typed after post_validation
Fixes#11381
