EIP-4881 was never correctly implemented, the `DepositTreeSnapshot`
structure has nothing to do with its actual definition. Reflect that
by renaming the type to a Nimbus-specific `DepositContractSnapshot`,
so that an actual EIP-4881 implementation can use the correct names.
- https://eips.ethereum.org/EIPS/eip-4881#specification
Notably, `DepositTreeSnapshot` contains a compressed sequence in
`finalized`, only containing the minimally required intermediate roots.
That also explains the incorrect REST response reported in #5508.
The non-canonical representation was introduced in #4303 and is also
persisted in the database. We'll have to maintain it for a while.
50a43f397f