persistency: reduce entries in decoding table to only those locations that are actually packed.

Remove redundant entries in the list of persisted locations used for the encoding/decoding.

This will effectively only allow references to containers which are actually packed and persisted, i.e. any reference to something which is not packed will become an error. Any such entries are presumably the result of a misconfiguration.

As a side-effect, having less (spurious!) entries in the table will make keys just a bit 'more stable' as there will be less (spurious) reasons for the contents of the table to change.