CONNECT Engine JDBC to support auto XA (Automatic Two Phase Commit of External Data)

Many JDBC drivers support XA (two phase commit aka Global Transactions or transactions that span multiple databases) including the MariaDB / MySQL JDBC drivers.

Config Option: CONNECT engine JDBC option XA=TRUE then, if the jdbc driver supports XA, to use XA symantics rather than auto commit in transactions involving connect engine JDBC.

SCENARIO: We might modify a local table L1 and a connect jdbc table from datasource A and another connect jdbc table from datasource B and finally a local table L2. If modifying L2 fails for some reason (e.g. SIGNAL from failure of a validation constraint within a trigger) the transaction can be rolled back which would rollback datasource A and datasource B changes.

This seems like it should be reasonable to implement since the JDBC driver architecture already supports the XA symantics as does MariaDB itself and its JDBC driver.

More info here: https://docs.oracle.com/cd/E13218_01/wlp/docs81/sp2/db/5XA.html

This can make MariaDB a hub for a federated transaction with all the benefits that implies.