Make reference checker more accurate

As of February this year we're using a reference checker to spot missing descriptors that reads files in recent/relay-descriptors/ and warns if too many referenced descriptors cannot be found.

However, our reference checker has been too noisy for me to pay much attention.

I didn't look at the logs in detail yet, but I came up with a possible improvement: we should only count an extra-info descriptor as missing if the referencing server descriptor is referenced from a consensus or vote. This is supposed to exclude all extra-info descriptors that are referenced from server descriptors uploaded to the directory authorities by bogus relays without also uploading the corresponding extra-info descriptors.

Maybe there are other tweaks that make these warnings more accurate and again worth checking by the operator.