fix(engine): wait for all state updates before returning state root task result

[fgimenez]

Depending on timing, the current condition for returning the state root from task can allow to return too early, when not all state updates have been processed https://github.com/paradigmxyz/reth/blob/main/crates/engine/tree/src/tree/root.rs#L437

These changes allow the state root task to detect when the sender in the state hook is dropped, so that no new state updates are expected. It should be used from the EngineApiTreeHandler like:

let state_hook_sender = root::StateHookSender { tx: state_root_tx.clone() };
let state_hook = move |state: &EvmState| {
    let _ = state_hook_sender.send(StateRootMessage::StateUpdate(state.clone()));
};       

[shekhirin]

LGTM. We can also make StateHookSender a thin wrapper over the Sender like pub(crate) struct StateHookSender(Sender<_>) with a Deref, but either works.

[fgimenez]

We can also make StateHookSender a thin wrapper over the Sender like pub(crate) struct StateHookSender(Sender<_>) with a Deref, but either works.

good point, done here a613633