feat: piecereader: Allow parallel access

[magik6k]

Related Issues

Closes #10532

Proposed Changes

• Allow pieceReader reader-getters to define exact read size
• Rewrite pieceReader.ReadAt to be called in parallel, while still caching small amounts ahead for parallel sequential reads
  • This is necessary, because carv1 reads byte-by-byte for the header/cid varints, saves ~9 requests per block read
• Kept the old more aggressive prefetching mechanism for ReadSeeker, which is only used by the indexing scan, and makes it wayy faster

Additional Info

• Make sure tests pass
• Test on a real setup, best in booster-bitswap
  • With a race detector

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