Sealing-free sector life extension

Authors: @Luca Nizzardo @Kubuxu

The sector lifetime limit exists to combat the possibility of broken PoRep or partially discarded replicas.

The brokenness of PoRep cannot be re-verified but proper storage or the replica can be verified by a PoSt style proof/Proof of Access just with a much higher challenge number.

This could in theory allow for some amount of sector lifetime extension beyond the 5yr maximum.