Better handling of issues caused by time traveling (DownloadTask) (DEV) #2018

* If maxExposureDetectionsPerDay is 0, return a saner value than 99 days.
maxExposureDetectionsPerDay is checked separately and will abort the download task if necessary.

* When checking whether the last detection was recent, add a sanity check that causes it to not skip, if the last detection was in our future.

Co-authored-by: Ralf Gehrer <ralfgehrer@users.noreply.github.com>