NV15: Speed up the migration

[arajasek]

The special Sectors migrating logic is as follows:

• For each miner, if the Head CID is found in the cache, use the cached value (same as any other actor)
• If not load the current state, do the quick steps like migrating MinerInfo
• Check the cache for the sectors AMT root -- if found, use the cached value and return back to miner migration
  • Note that this step will cover many hundreds of thousands of "empty" miners in the state tree
• If not found, check the cache to see if a previous sectors AMT has been cached for this miner. If not found, manually migrate all sectors, and insert the incoming and outgoing AMTs into the cache (specific to the miner)
• If the cache does have a previous sectors AMT cached for this miner, load it and its corresponding migrated AMT. Diff the previous AMT against the current one, mutating the previous migrated AMT appropriately. Update the cached AMT roots.

[codecov-commenter]

Codecov Report

Merging #1548 (7140d56) into master (b1dfad7) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1548   +/-   ##
======================================
  Coverage    69.0%   69.0%           
======================================
  Files          73      73           
  Lines        8798    8798           
======================================
  Hits         6079    6079           
  Misses       1858    1858           
  Partials      861     861           

[arajasek]

Rebased on top of latest master (revert of deal label PR)

[arajasek]

Testing update: Ran on mainnet state with and without the cache, same result.

Next steps are a unit test coming from @geoff-vball and maybe writing an unmigration, and testing that.

[arajasek]

Landing this, thanks Steb!