sequtils: robust cnum generation

* Implements more robust cnum generation by not assuming that, if only
  1 cnum is found in the seqSTORE with a given base, it doesn't already
  have version as a suffix.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Merge pull request #62 from kaplun/fix-cnum-in-prod

sequtils: robust cnum generation