non-ftpsync mirrors shouldn't be second-class citizens if they meet the criteria

[rgeissert]

There are currently some requests that are forced to be served by mirrors that use ftpsync. This is to guarantee that there will not be inconsistencies caused by some index files being synchronised too early.
Since there are quite some mirrors that don't use ftpsync, and some of them do keep up with the recommended rsync settings and other changes introduced in ftpsync, it would be ideal not to treat them as second-class citizens.

Continuously performing all sorts of checks to determine if they don't follow the recommendations is doomed to fail. Perhaps a new field could be introduced in the trace file that states which "features"/changes they have been updated to.

For instance, mirrors that correctly sync the InRelease file in the second stage could include:
Revision: InRelease

The absence of the field would indicate that such mirror should not be used to serve InRelease files.

Similarly, for the translation files issues:
Revision: i18n

Whether or not this field should be included in ftpsync-generated trace files should be considered. For consistency, it probably should.