Update VT goroutines errs & Update Pack&UnpackSibl

- Update VT goroutines errs to avoid race condition
- Update pack & unpack siblings to use 2-byte for full length & bitmap
bytes length
- Add check in UnpackSiblings to avoid panic

tree_test.go

@@ -311,7 +311,7 @@ func TestGenProofAndVerify(t *testing.T) {
 	c.Assert(err, qt.IsNil)
 	defer tree.db.Close() //nolint:errcheck
 
-	bLen := tree.HashFunction().Len()
+	bLen := tree.HashFunction().Len() - 1
 	for i := 0; i < 10; i++ {
 		k := BigIntToBytes(bLen, big.NewInt(int64(i)))
 		v := BigIntToBytes(bLen, big.NewInt(int64(i*2)))