fix: parallelise loading of dag-pb links when exporting

[achingbrain]

A polishing of #237. Uses it-parallel to load a whole list of children of a DAG node in parallel rather than one at a time. Makes fetching large files much faster.

From the js-ipfs interop suite:

Before:

  exchange files
    go -> js
      cat file
        ✔ go -> js: 67.1 MB (25659ms)
    go -> go2
      cat file
        ✔ go -> go2: 67.1 MB (19835ms)
    js -> go
      cat file
        ✔ js -> go: 67.1 MB (17407ms)
    js -> js2
      cat file
        ✔ js -> js2: 67.1 MB (20237ms)

After:

  exchange files
    go -> js
      cat file
        ✔ go -> js: 67.1 MB (15328ms)
    go -> go2
      cat file
        ✔ go -> go2: 67.1 MB (18536ms)
    js -> go
      cat file
        ✔ js -> go: 67.1 MB (17211ms)
    js -> js2
      cat file
        ✔ js -> js2: 67.1 MB (12124ms)

[github-actions]

🎉 This PR is included in version ipfs-unixfs-exporter-v8.0.2 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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