[release/1.4.18] trie: fix regression that linked all downloaded nodes together

The trie sync code links subtries using pointers into node structs.
Since commit 40cdcf1 nodes are no longer copied when unpacking from
an interface value, causing all nodes to get linked up as the sync
progresses. Fix it by breaking the pointer chain with an explicit copy.

(cherry picked from commit 2cd7a03)

Author: fjl

trie/sync.go

@@ -213,11 +213,13 @@ func (s *TrieSync) children(req *request) ([]*request, error) {
 
 	switch node := (*req.object).(type) {
 	case *shortNode:
+		node = node.copy() // Prevents linking all downloaded nodes together.
 		children = []child{{
 			node:  &node.Val,
 			depth: req.depth + len(node.Key),
 		}}
 	case *fullNode:
+		node = node.copy()
 		for i := 0; i < 17; i++ {
 			if node.Children[i] != nil {
 				children = append(children, child{