Fix the model implementation of `mUnfetchedPoolMetadataRefs`.
The property expects that recent fetch attempts are excluded from the list returned by `unfetchedPoolMetadataRefs`. We adjust the model to satisfy the property. Note that the property coverage here isn't necessarily as complete as it could be. We'd ideally want to test this pool DB function in conjunction with other pool DB functions as part of a quickcheck-state-machine style test.