Correctly check instances that are run on top of real mirrors

[rgeissert]

If a mirror runs a redirector on top of its real copy, the checker might be redirected to a different mirror when the trace file is requested. This will result in the mirror being disabled.
If the checker is aware of mirrors that run the redirector, it could download the file from serve/ instead and correctly monitor the mirror.

This most likely requires a new field in the master list.