feat: Save specific date for manual results

We cannot rely on the same date for trips and manual data because we
might end up in this (unlikely, though) situation:

1. Some manual data is sent to the trace server
2. The konnector is run: it finds trips
3. A new trip is sent on the trace server at the same time
4. A new manual data is sent on the trace server at the same time
5. The konnector finds the new manual data with a more recent date than the last
found trip
6. The konnector save the date of the new manual data
7. The new trip won't be found at the next run