perf(trie): parallel rlp node updates in sparse trie

[shekhirin]

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Makes the RevealedSparseTrie::rlp_node take a non-mutable reference to self and return the list of updates that are need to be applied to the trie nodes. This allows us to run multiple rlp_node in parallel in the RevealedSparseTrie::update_rlp_node_level, because we gather the nodes at the provided level and calculate their hashes independently from each other.