Reject obsolete router/extrainfo descs more quietly.

Thanks to proposal 315 / ticket #30132, more fields are now required in these documents. But ancient Tors that try to upload obsolete documents were causing the authorities to log warnings about missing fields, and to do so very spammily.

We now detect the missing fields before tokenizing, and log at debug. This is a bit of ugliness, but it's probably a safer choice than making all unparseable-desc warnings into debug-level logs.

I'm looking at identity-ed25519 in extrainfos and proto in routerdescs because they were (I believe) the latest-added fields in Tor's history: any Tor that lacks them will also lack the other newly required fields.

Fixes bug #40238 (closed); bugfix on 0.4.5.1-alpha.