Introduce FIP to exempt Window PoSts from BaseFee burn

[Stebalien]

Summary: Exempt direct SubmitWindowedPoSt messages that execute successfully from base-fee burn (i.e., don't burn baseFee*gasUsed).

If a miner sends a direct on-chain message to a miner actor's "SubmitWindowedPoSt" method, and the message executes successfully, immediately "refund" all the burned gas, instead of just refunding the overestimation. However, overestimation penalties, gas premium, etc. still apply.

Rendered

Motivation:

This proposal is a short-term stopgap to reduce the impact of rising base fee on continuously proving existing storage. In principle, BaseFee*GasUsage is burned to compensate the network for the resources consumed by messages and ensure incentive alignment. However, SubmitWindowPoSt is a required message to continue mining operations in Filecoin.

• Long-term solutions for reducing the cost of Window PoSt include Off-chain Window PoSt verification #42.
• Long-term solutions for reducing chain congestion include Reduce congestion via Batching ProveCommits #49, Reduce congestion via Aggregating ProveCommits via Inner Product Pairing #50.

Unfortunately, such long-term solutions are complex and cannot be properly implemented and tested on the required timescale (before the holidays). This proposal is a short-term mitigation until those solutions are ready.

[arajasek]

Proposal looks good.