`SyncCommitteeMsgPool` grouped messages by their `beacon_block_root`.
This is problematic around sync committee period boundaries and forks.
Around sync committee period boundaries, members from both the current
and next sync committee may sign the same `beacon_block_root`; mixing
the signatures from both committees together is a mistake. Likewise,
around fork transitions, the `signing_root` changes, so those messages
also need to be segregated.
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