Improve message tree task message model

[mattsse]

Currently we can't handle engine requests and persistence events concurrently because they use two different channels

so we only proceed with handling persistence tasks after we received a new engine message.

these should use a single channel, this means we need to unify both channels and use a superset of event variants

reth/crates/engine/tree/src/tree/mod.rs

Line 395 in 7864ae0

incoming: Receiver<FromEngine<BeaconEngineMessage<T>>>,

so that this is returned over the unified channel

reth/crates/engine/tree/src/tree/mod.rs

Lines 1716 to 1718 in 7864ae0

	/// Receiver end of channel where the result of the persistence task will be
	/// sent when done. A None value means there's no persistence task in progress.
	rx: Option<oneshot::Receiver<B256>>,