As a result of #45689 snapshot finalization started to
take significantly longer than before. This may be a
little unfortunate since it increases the likelihood
of failing to finalize after having written out all
the segment blobs.
This change parallelizes all the metadata writes that
can safely run in parallel in the finalization step to
speed the finalization step up again. Also, this will
generally speed up the snapshot process overall in case
of large number of indices.
This is also a nice to have for #46250 since we add yet
another step (deleting of old index- blobs in the shards
to the finalization.
3d23cb44a3