Merge tag 'sparks-1.20.3' into develop

1.20.3
Allow the batcher thread class to delay it's start.
add rabbitmq on OSX (Django tasks) and make runable install components.
revert to parallel=False in mails, which is expected while running via celery/RQ. The `parallel` mode is a comfort option while developing without a task system, but should be considered as a temporary measure unless you install one, which is quite "standard" or required nowadays.
bump version in setup.py
Merge branch 'release/1.20.4'