dump: Parallelize loading large files

[MichaelEischer]

What does this PR change? What problem does it solve?

Improve dump performance for large files by loading their blobs concurrently using repo.Connections() goroutines.

The largest part of the code changes in here is actually a modification of the bloblru cache. There's now a new GetOrCompute method that ensures that if multiple goroutines try to get the same blob that is not yet in the cache, that the blob will only be downloaded once.

I've initially tried to use a sync.Pool to reuse byte slices, however, that turned out to be rather complicated to get right when using the bloblru cache. A byte slice may only be reused if no other goroutine can have a reference to it. With the current interface this is next to impossible to guarantee.

Was the change previously discussed in an issue or on the forum?

Basic variant as discussed in #3406 .

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• [ ] I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[MichaelEischer]

LGTM