Implement cursor support

### Issue description
In some cases it might be undesirable to fetch a potentially huge resultset at once.

MySQL provides cursors for that in the binary protocol (prepared statements).

First, a cursor flag has to be set in [`COM_STMT_EXECUTE`](https://dev.mysql.com/doc/internals/en/com-stmt-execute.html), then the result chunks can be requested with the [`COM_STMT_FETCH` command](https://dev.mysql.com/doc/internals/en/com-stmt-fetch.html).

Initially, we will probably only support forward-only and read-only cursors. However the question remains, trough what interface we make this feature available.