trie: cache collapsed tries node, not rlp blobs (#16876)

The current trie memory database/cache that we do pruning on stores
trie nodes as binary rlp encoded blobs, and also stores the node
relationships/references for GC purposes. However, most of the trie
nodes (everything apart from a value node) is in essence just a
collection of references.

This PR switches out the RLP encoded trie blobs with the
collapsed-but-not-serialized trie nodes. This permits most of the
references to be recovered from within the node data structure,
avoiding the need to track them a second time (expensive memory wise).

trie/trie.go

@@ -433,12 +433,10 @@ func (t *Trie) resolveHash(n hashNode, prefix []byte) (node, error) {
 	cacheMissCounter.Inc(1)
 
 	hash := common.BytesToHash(n)
-
-	enc, err := t.db.Node(hash)
-	if err != nil || enc == nil {
-		return nil, &MissingNodeError{NodeHash: hash, Path: prefix}
+	if node := t.db.node(hash, t.cachegen); node != nil {
+		return node, nil
 	}
-	return mustDecodeNode(n, enc, t.cachegen), nil
+	return nil, &MissingNodeError{NodeHash: hash, Path: prefix}
 }
 
 // Root returns the root hash of the trie.