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/node.go

@@ -47,9 +47,22 @@ type (
 	valueNode []byte
 )
 
+// nilValueNode is used when collapsing internal trie nodes for hashing, since
+// unset children need to serialize correctly.
+var nilValueNode = valueNode(nil)
+
 // EncodeRLP encodes a full node into the consensus RLP format.
 func (n *fullNode) EncodeRLP(w io.Writer) error {
-	return rlp.Encode(w, n.Children)
+	var nodes [17]node
+
+	for i, child := range n.Children {
+		if child != nil {
+			nodes[i] = child
+		} else {
+			nodes[i] = nilValueNode
+		}
+	}
+	return rlp.Encode(w, nodes)
 }
 
 func (n *fullNode) copy() *fullNode   { copy := *n; return &copy }