Key single lookups by block root instead of id

[realbigsean]

Description

Follow up from #5655

A lot of the time we're looking for duplicate single lookup requests based on block roots, it makes sense to just store with block root as a key rather than Id.

[realbigsean]

stale

[dapplion]

Now I'm convinced we should NOT implement this. If a lookup fails, we can have two distinct lookups for the same block root. To debug lookups it's important to track them individually; not having a unique ID will make debugging the logs unnecessarily hard.