PostRR_update(): Fixed handling of multiple columns in the same table.

Previously, after updating the first column, the next columns would already
find a matching timeslice (by timestamp), thus updating the old value.

Now, all old data will simply be removed (for timeslices which are strictly
older than the new timestamp (by timestamp)). This ensures that all data
related to the old timestamp is removed.

Note: However, this *might* destroy unrelated data as well in case a user
stores additional information in that table. For now, I will simply ignore
that as I don't see any reasonable justification for doing so ;-)