Populating the index on snapshot import

[jennijuju]

          Great work here. Some loose ends we need to tie up in subsequent PRs:

• Populating the index on snapshot import.
• Mpool index management (see threads above).
• add a lotus-shed cmd to enable node operators to back porting the ID indexes maps that they are missing. #10075

Originally posted by @raulk in #9965 (review)

[magik6k]

#10119