Before declaring a chunk invalid and needing download from chifra init, write its header back in and see if that corrects the problem

[tjayrush]

About 90% of all the chunks in version v0.40.0 only differed by having an incorrect header.

Also, if there's a local IPFS node, we can actually do the verification after download (or updating the header) and that would short cirucut any other checks.

Same with Bloom filters.