Mark removed packages automatically

[deathaxe]

This commit fixes mark_removed task, by really marking only those packages as removed in database, which don't have any valid download source anymore and are not already marked missing - possibly due to networking issues.

Run cleanup regularly via cron, like rename cleanups.

The goal is to stop delivering packages to clients, which have been removed from package_control_channel as associated URLs/repos might be taken over by bad actors delivering malware.