Correct reward denominator in op pool

[michaelsproul]

Issue Addressed

Closes #5016

Proposed Changes

The op pool was using the wrong denominator when calculating proposer block rewards! This was mostly inconsequential as our studies of Lighthouse's block profitability already showed that it is very close to optimal. The wrong denominator was leftover from phase0 code, and wasn't properly updated for Altair.

Additional Info

After this change there is better agreement between /lighthouse/analysis/block_rewards and the standard rewards API, however they are not yet calculating identical results because of how the per-attestation rewards are rounded in /lighthouse/analysis/block_rewards. A more extensive refactor of the max cover code to divide by the denominator after grouping attesters by attestation would fix this discrepancy.

E.g.

slot 8161659: block from lighthouse-subscribe-none with 64 attestations & purported reward 42231047 gwei
slot 8161658: canonical <=> lighthouse-subscribe-none distance: 4636
slot 8161658: canonical block is likely Lighthouse@4636 (two closest match)
rewards from lighthouse-subscribe-none: 42227080 gwei (att: 40729402 gwei)
most profitable block from ["lighthouse-subscribe-none"]

I'm not sure if this change is worth making yet.