`mixer build delta-packs` can break delta creations when using `--previous-versions`

[phmccarty]

When running mixer build delta-packs --previous-versions=N, packs will be generated for the previous N versions, as expected. But the delta list and delta creations for each previous version are processed independently, and in parallel, which often leads to unexpected bsdiff delta creation/application failures when delta lists overlap (i.e. contain duplicates if the lists were combined).

Instead, I think it would be better for mixer to (a) compute the delta list for the previous N versions (de-duplicated), (b) create all deltas from the delta list in parallel, and finally (c) create the delta packs.

The entrypoint for this process is:

mixer-tools/builder/builder.go

Line 591 in 8e8ba2d

func (b *Builder) BuildDeltaPacksPreviousVersions(prev, to uint32, printReport bool) error {