Commit() function can not distinguish whether the transaction has been sent

### Issue description
when I call func (tx *mysqlTx) Commit() (err error) ，if connection has problem it return an ErrInvalidConn


but if current connection is fine it call exec


and in exec it call readResultSetHeaderPacket after the transaction sent to db successfully, and readResultSetHeaderPacket call readPacket() and it also may lead to ErrInvalidConn


thus I can't distinguish whether the transaction has been sent, it lead to one transaction sent twice and data not correct. Is it possible to add a new error to distinguish this two cases?

### Example code
```go
If possible, please enter some example code here to reproduce the issue.
```
err = tx.Commit(ctx)
if err != nil {
xlog.Errorf(ctx, "%s commit transaction failed, table:%v err:%v", fun, table, err) ...

### Error log
```
***err:invalid connection
```

### Configuration
*Driver version (or git SHA):*

*Go version: go1.12 linux/amd64

*Server version: tidb

*Server OS: redhat7.2