Fix bug in aggregator verification

[dapplion]

Motivation

Note: must use bytesToBigInt() otherwise a JS number is not able to represent the latest digits of the remainder, resulting in 14333559117764833000 for example, where the last three digits are always zero. Using bytesToInt() may cause isSelectionProofValid() to always return false.

Description

• Use bytesToBigInt()
• Use correct constant

[codeclimate]

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[dapplion]

This bug slipped through because it's not covered by spec tests. This logic is purely p2p, so:

• it's not covered by any spec tests, nor unit tests
• it won't cause a consensus failure in our nodes

To prevent this from happening again we should move all logic not used in the state transition from the beacon-state-transition package. Put it somewhere else (like in the lodestar package) and heavily test it.

[twoeths]

seems like we have the other isSyncCommitteeAggregator in packages/validator/src/util/aggregator.ts which uses BigInt there, we should deduplicate it

[wemeetagain]

Good catch!