Expression-based client facade moved to the BackgroundClass
JobMetadata -> JobInvocationData
BREAKING: Removing the old client (old jobs are still supported)

Old client is very different from the new client. It's easier to get rid
of it than to simultaneously support two clients. However, server is
still able to process jobs created by the old client. So, users need to
perform transition to the new client, while their old jobs will be
processed anyway.