script/gen-corpus: cleanup and improve readability and performance

- gen-corpus cleanup missed removing the temporary directory.
- improve it a bit for readability
- import.sh script was slow, improve performance by using less forks and
  batch processing

Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>

Merge pull request #66158 from xxhdx1985126/wip-seastore-minor-perf-issue

crimson/os/seastore/epm: avoid unnecessary container copies

Reviewed-by: Matan Breizman <mbreizma@redhat.com>
Reviewed-by: Samuel Just <sjust@redhat.com>