Run the per-file mbsdiff/xz work in make_incremental_update.sh and
make_full_update.sh through a parallel job pool, largest files first, and
assemble the manifests serially afterward, producing byte-identical MARs.
Set UPDATE_PACKAGING_JOBS to override the number of parallel jobs.
The old updater used MARs with bzip2-compressed files, but Mozilla switched to
xz compression, so we generate xz-based MARs using the current MAR tools and
`make_(full|incremental)_update.sh` scripts and add a script to generate
bzip2-based MARs to serve to pre-7.0 builds.
