Extend zoekt node unclaimed_storage_bytes

When a zoekt index creation is scheduled for a zoekt node, it also reserves a portion of free space on the node (deduced as percentage of project repos).

These reserved storage is taken into account in order_by_unclaimed_space_desc when returning ordered nodes suitable for scheduling zoekt/knowledge graphs DBs. We should update order_by_unclaimed_space_desc to take into account also estimated reserved space for knowledge graphs. This should be ideally generic enough so it can be easily extended in future also for other similar services (other than knowledge graph and zoekt).