specs: consider limiting record-key length to 64 (not 512)

[bnewbold]

Raised by @dholms in bluesky-social/atproto#1738 (comment)

I'm generally open to this. The main consideration to me is what tricks we might want to use to stuff other identifiers in to record keys to make lookups easier. Eg, CIDs if the record is metadata about a blob, DID if metadata about accounts (maybe we should have done that for follows!), etc.