fix(root): spawn state root task only if host has enough parallelism

[shekhirin]

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

Lines 55 to 65 in 4822b7d

	/// Determines if the host has enough parallelism to run the state root task.
	///
	/// It requires at least 5 parallel threads:
	/// - Engine in main thread that spawns the state root task.
	/// - State Root Task spawned in [`StateRootTask::spawn`]
	/// - Sparse Trie spawned in [`run_sparse_trie`]
	/// - Multiproof computation spawned in [`MultiproofManager::spawn_multiproof`]
	/// - Storage root computation spawned in [`ParallelProof::multiproof`]
	pub(crate) fn has_enough_parallelism() -> bool {
	std::thread::available_parallelism().map_or(false, |num| num.get() >= 5)
	}