Add support for "one-shot" sources

[nhairs]

Some sources (e.g. source.file.DirectoriesAndFiles) will run out of jobs and throw StopIteration. Whilst this is correctly caught and the SourceWorker will shutdown we should monitor all the source workers and if they all shutdown then we should shutdown the whole application.

This might need a new state as the sources should not be shutdown/cleaned up until after the sink workers have shutdown as the sources will need to ack the jobs.