Added index to DownloadStatistics.fID for query where + order by timestamp, and for potential join to the files table

Added index to DownloadStatistics.timestamp for select all order by timestamp query

Added index to DownloadStatistics.fvID for potential join to FileVersions table

Added index to DownloadStatistics.uID for potential join to Users table

Added index to DownloadStatistics.rcID for potential join to Collections? table