In order to avoid full replays when validating attestations hailing from
untaken forks, it's better to keep shufflings separate from `EpochRef`
and perform a lookahead on the shuffling when processing the block that
determines them.
This also helps performance in the case where REST clients are trying to
perform lookahead on attestation duties and decreases memory usage by
sharing shufflings between EpochRef instances of the same dependent
root.
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