sync: Send already connected peers to new subscribers (#7011)
Introduce `SyncEvent::InitialPeers` message sent to new subscribers to
allow them correctly tracking sync peers. This resolves a race condition
described in
https://github.com/paritytech/polkadot-sdk/issues/6573#issuecomment-2563091343.
Fixes https://github.com/paritytech/polkadot-sdk/issues/6573.
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Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <[email protected]>