sequtils: more 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.  (PR #2119)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>