trie: ensure resolved nodes stay loaded

Commit 40cdcf1183 broke the optimisation which kept nodes resolved
during Get in the trie. The decoder assigned cache generation 0
unconditionally, causing resolved nodes to get flushed on Commit.

This commit fixes it and adds two tests.

trie/proof.go

@@ -101,7 +101,7 @@ func VerifyProof(rootHash common.Hash, key []byte, proof []rlp.RawValue) (value
 		if !bytes.Equal(sha.Sum(nil), wantHash) {
 			return nil, fmt.Errorf("bad proof node %d: hash mismatch", i)
 		}
-		n, err := decodeNode(wantHash, buf)
+		n, err := decodeNode(wantHash, buf, 0)
 		if err != nil {
 			return nil, fmt.Errorf("bad proof node %d: %v", i, err)
 		}