Make FetchUselessDescriptors fetch all consensus flavours

FetchUselessDescriptors 1 used to imply UseMicrodescriptors 0, but due to legacy/trac#6769 (moved), it doesn't in 0.3.0 and later.

Therefore, clients that want to download a full consensus have to explictly set UseMicrodescriptors 0.

We should document this in the man page and probably the changelog summary, as it is a breaking change for many configs that used to obtain a full consensus.

(Alternately, we could fix this bug by FetchUselessDescriptors download a full consensus, even on clients, and document that behaviour.)

Discovered when running a test bandwidth authority - see legacy/trac#20621 (moved).