Implement SDP resource management

Amount of available buffer space gets registered as a resource in the configuration database Might just boil down to checking "df", but... Should clearly not change dynamically since a processing block writes data within its reservation, so the proper definition would be either free space MINUS data product sizes of processing blocks PLUS processing block reservations, OR total available space MINUS processing block reservations MINUS sizes of non-processing-block data products Allow processing scripts to add requests for buffer space (associated with stages) Receive processing block would use projected observation length and other parameters from the execution block (polarisations, channels, receptors) to approximate size needed. Implement simple mechanism for these requests to be granted Processing controller (or separate service?) would monitor resources + requests and assign Implement mechanism for resource request to fail that is visible to operators Ideal solution would be for AssignResources to fail already, but that would require looking ahead Easier would be to just have real-time processing blocks fail (by some mechanism) if they don't get their resources assigned