client can use bad WUs
Imported issue: Initially reported by @zimmerma in https://gforge.inria.fr/tracker/?group_id=2065&aid=21811
when running two factorizations with numbers of the same size, with --dldir=$HOME/dldir --nosha1check, the WUs of the 1st one are left in DLDIR, and for the 2nd factorization, they are taken by the client instead of the new ones. This happens first in polyselect stage 2 (root optimization). Then cado-nfs reports that all size optimized polynomials are for the wrong number, which results in no polynomial being found.