zstd:chunked convert_images: Do the conversion in `GetDiffer`?

[mtrmac]

c/image calls chunked.GetDiffer immediately followed by store.ApplyDiffWithDiffer ~ differ.ApplyDiff.

Currently convert_images happens in the latter call; but that means it happens with the layer store locked for writing. It seems to me that moving the conversion to the GetDiffer phase would increase opportunities for concurrency, especially for slow networks. (And it would probably simplify the data stored in chunkedDiffer and the ApplyDiff implementation a bit.)

Cc: @giuseppe . This is certainly not urgent, and might even not be desirable to do right now — but I hear performance of this code path is a concern, so this might be worth considering.