Fix bugs in GDS vertex property scanning

[benjaminwinger]

Based on #4416

I fixed some edge cases in the NodeGroup::scan variant used by GDS vertex property scanning, and also fixed handling scans that run across node group boundaries.

I renamed NodeGroupScanState::numScannedRows to nextRowToScan since they are functionally equivalent and nothing was ever using numScannedRows as a size, just as an indication of the next row to scan. There was also a field called nextRowToScan already in CSRNodeGroupScanState, so that was just removed and the other field is now used in its place.

[benjaminwinger]

Do we not have CI for PRs which don't target the master branch? This was passing all tests locally (notably including the two tests which were failing for me initially on #4416).