Update Sparse Trie subtries in parallel

[Rjected]

Mostly extracting from this #13251

We should be able to update independent subtries in parallel, but right now we cannot because this all takes a mutable reference. All node updates are currently sequential.

We see lots of sequential rlp_node work being done in profiles here:

If we can make rlp_node take &self:

reth/crates/trie/sparse/src/trie.rs

Line 754 in bb5ca25

pub fn rlp_node(

Then, it should be possible to parallelize calculate_below_level.