Scheduler request parameter to indicate relevant start time

[dkastl]

When providing real-time scheduling (see #16 ) a project's operation may already have started when new jobs/shipments are posted.

To be able to handle these new requests while other jobs/shipments have been already completed or are currently in progress, it's necessary to be able to provide a timestamp for example, from which scheduling will be done.

Any job/Shipment before that time will not be considered.

New requests must start past this timestamp, otherwise they will be rejected.