backup /var/lib/rpm at update (bnc#857639)

- ... to prevent from accidental corruption
- switch from bzip2 to gzip compression (slightly less compression
  ratio, but much faster), use parallel gzip (pigz) for even
  faster backup
- 3.1.21
Merge pull request #32 from yast/rpm_db_backup

backup /var/lib/rpm at update (bnc#857639)