Add generate_request functions (stmt_execute and bulk_execute) to internal API

When using C/C within python, every api call needs to be encapsulated by BEGIN/END Threads, e.g.

Py_BEGIN_ALLOW_THREADS()

rc= mysql_stmt_execute(self->stmt);

Py_END_ALLOW_THREADS()

This causes a lot of overhead, in case of callback function for filling values was specified: Whenever calling python code inside mysql_stmt_execute() we need to call Py_END_ALLOW_THREADS and Py_BEGIN_ALLOW_THREADS() again.

Instead of it would be much easier to have a call which fills the buffer and another call which executes the stmt:

buffer= self->connection->mysql->methods->generate_execute_request(self->stmt, &buflen)

PY_BEGIN_ALLOW_THREADS()

rc= self-connection->mysql->methods->send_command(COM_STMT_EXECUTE, buffer, buflen, ...)

PY_BEGIN_END_THREADS()

PY_END_ALLOW_THREADS()