Default implementation of the delivery manager.
Internal interface for testing. See
nti.webhooks.testing.SequentialExecutorServicefor the alternate implementation.
Submit a job to run. Execution may or may not commence in the background. Tracks tasks that have been submitted and not yet finished.
Wait for all tasks that have been submitted but not yet finished to finish.
submit, wait for them all to finish. If any one of them raises an exception, this method should raise an exception.
Stop accepting new tasks.