Neutron: Off-chain miner state

The Neutron proposal for exponential growth of Filecoin storage is proposed in FIPs#119 (July 2021) and described in detail in

https://docs.google.com/document/d/1f1ECHRJTWkeXOjlN5f_NzV3qIsp-tnPJP0kYX0nextE/edit#heading=h.fe8v5avvtget

Implementation was paused after the Hyperdrive network upgrade, after which chain capacity was expanded well beyond the currentc growth rate, and growth rate stopped increasing.