Optimize the full unseal snap deals sectors

[geoff-vball]

Checklist

• This is not a new feature or an enhancement to the Filecoin protocol. If it is, please open an FIP issue.
• This is not a new feature request. If it is, please file a feature request instead.
• This is not brainstorming ideas. If you have an idea you'd like to discuss, please open a new discussion on the lotus forum and select the category as Ideas.
• I have a specific, actionable, and well motivated improvement to propose.

Lotus component

• lotus daemon - chain sync
• lotus miner - mining and block production
• lotus miner/worker - sealing
• lotus miner - proving(WindowPoSt)
• lotus miner/market - storage deal
• lotus miner/market - retrieval deal
• lotus miner/market - data transfer
• lotus client
• lotus JSON-RPC API
• lotus message management (mpool)
• Other

Improvement Suggestion

Currently in order to regenerate the sector key for a snap deal sector requires a call to SealPreCommit2 which is expensive and shouldn't be necessary. Right now it is necessary because it generates the p_aux cache file that is required by the call to DecodeFrom. We should preserve p_aux when we call FinalizeReplicaUpdate in order to make unsealing the sector less resource intensive.

There may be other optimizations to be made to eliminate other unnecessary work in the ffi calls.

#8478 (comment)

[geoff-vball]

This is addressed by #8478