Align SectorStateMgr and gql/resolver_piece #1506
Description
At the moment resolver_piece.go is calculating itself whether a piece is unsealed or not, via its sealStatus function.
It should be refactored to rely on SSM, and SSM should be refactored to check whether an unseal copy is real and working, and not corrupt. At the moment SSM just returns whatever StorageList returns and this could be misleading.