It is necessary that each archive ID caching function call this method for each
unique 'done' flag it encounters, since the getArchiveIds function determines
whether archiving should be launched based on whether $this->idarchives has a
an entry for a specific 'done' flag.
If this function is not called, then periods with no visits will not add
entries to the cache. If the archive is used again, SQL will be executed to
try and find the archive IDs even though we know there are none.