fix(webrtc): warp sync slow initialization and webrtc handshake bug

[gab8i]

The following PR just extract work already done within paritytech/smoldot-archive#2222.

The fixes are:

• Solving a deadlock within the multistream-select task loop
• Initializing warp sync faster by avoiding the need to wait for Grandpa notifications. This is especially needed by WebRTC because data channel creation introduces latency, and thus not waiting for the channel to open saves time during syncing.

[dmitry-markin]

Substream waking in WebRTC looks good.

The idea behind optimistically targeting warp sync at the best block and not finalized one is also good, but we need to rename finalized_block_height to something like optimistic_finalized_block_height, or a better name, so future code doesn't rely on this value as actually being finalized.

Also, the comment at warp_sync.rs:768 likely needs updating and the corresponding logic checked for correctness.

[lexnv]

Should we move the write_ready.listen() outside the Box::pin future?

Currently, the write listener is registered once the the when_substreams_rw_ready is firstly polled.
This means we could arrive in a situation where .notify(usize::MAX); is called before registering the listener call.

For this to happen we need to:

• receive WakeUpReason::CoordinatorMessage(message)
• message_sending.as_ref().as_pin_ref().is_none() should have some data to effectively take the None path and ignore the when_substreams_rw_ready.select_next_some()

Moving the listener above Box::pin solves this regardless of which states call when_substreams_rw_ready.poll_next and avoids the problem entirely

[lexnv]

LGTM!