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

### Notes ###

- `pigz` is included in SLE12 and it's used by `dracut` which is needed by `mkinitrd`, it's a core package and should be supported well
- I did some research about speed vs. backup size, see [the table](https://docs.google.com/spreadsheets/d/1g0pTSd_ZCnwBaUGKX4sRUc1URtfe49_Ay7eYRgcgDbs/edit?usp=sharing)
- tested in update from SLES11-SP3 (`pigz` was added to inst-sys by `dud=` option)