Keep track of same files which are part of different releases

SoarSender.sent (list_sent_files.csv) is indexed by filename, so there can be only one row per filename, even if the same filename is present in different releases. So only the first release tag for each filename is listed.

The table should be indexed by (release_tag, filename), so that a filename appearing in different releases appears as many times as the number of releases (even if this means that filename, level, spiobsid, rasterno, version, file_size will be duplicated).

sent_date will be different in different releases although the file has been sent to SOAR only once. When checking files presence on SOAR, only the first of the dates for the file in different releases should be used (filenames will have to be deduplicated first).