feat: shed: Online export-car

[magik6k]

Proposed Changes

Make lotus-shed export-car work on online nodes

Additional Info

$ ./lotus state get-actor f05
Address:        f05
Balance:        134827.875759811187127269 FIL
Nonce:          0
Code:           bafk2bzacediohrxkp2fbsl4yj4jlupjdkgsiwqb4zuezvinhdo2j5hrxco62q (fil/8/storagemarket)
Head:           bafy2bzacecqryedsccndhepiovpm7somjzblvwbb5gdeq2wqy6tbi52yhyejg

$ ./lotus-shed export-car actor.car bafy2bzacecqryedsccndhepiovpm7somjzblvwbb5gdeq2wqy6tbi52yhyejg
done 1.907 GiB

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green