Generating txn id.

Related to https://github.com/mariadb-corporation/mariadb-columnstore-cmapi/pull/131#discussion_r746404541

In some exception handling code we have places where trying to use txn id in exception msg.
But it generates inside another function and returns only on success.
Here we should generate the txn id to be distributed to nodes separately so that there will be an id that can be used in exception msg.