trie: implement NodeBlob API for trie iterator

This functionality is needed in new path-based storage scheme, but
can be implemented in a seperate PR though.

When an account is deleted, then all the storage slots should be
nuked out from the disk as well. In hash-based storage scheme they
are still left in the disk but in new scheme, they will be iterated
and marked as deleted.

But why the NodeBlob API is needed in this scenario? Because when
the node is marked deleted, the previous value is also required to
be recorded to construct the reverse diff.

trie/iterator_test.go

@@ -521,3 +521,54 @@ func TestNodeIteratorLargeTrie(t *testing.T) {
 		t.Fatalf("Too many lookups during seek, have %d want %d", have, want)
 	}
 }
+
+func TestIteratorNodeBlob(t *testing.T) {
+	var (
+		db      = memorydb.New()
+		triedb  = NewDatabase(db)
+		trie, _ = New(common.Hash{}, triedb)
+	)
+	vals := []struct{ k, v string }{
+		{"do", "verb"},
+		{"ether", "wookiedoo"},
+		{"horse", "stallion"},
+		{"shaman", "horse"},
+		{"doge", "coin"},
+		{"dog", "puppy"},
+		{"somethingveryoddindeedthis is", "myothernodedata"},
+	}
+	all := make(map[string]string)
+	for _, val := range vals {
+		all[val.k] = val.v
+		trie.Update([]byte(val.k), []byte(val.v))
+	}
+	trie.Commit(nil)
+	triedb.Cap(0)
+
+	found := make(map[common.Hash][]byte)
+	it := trie.NodeIterator(nil)
+	for it.Next(true) {
+		if it.Hash() == (common.Hash{}) {
+			continue
+		}
+		found[it.Hash()] = it.NodeBlob()
+	}
+
+	dbIter := db.NewIterator(nil, nil)
+	defer dbIter.Release()
+
+	var count int
+	for dbIter.Next() {
+		got, present := found[common.BytesToHash(dbIter.Key())]
+		if !present {
+			t.Fatalf("Miss trie node %v", dbIter.Key())
+		}
+		if !bytes.Equal(got, dbIter.Value()) {
+			t.Fatalf("Unexpected trie node want %v got %v", dbIter.Value(), got)
+		}
+		count += 1
+	}
+	if count != len(found) {
+		t.Fatal("Find extra trie node via iterator")
+	}
+}