Add "CONVERT" option to "XA RECOVER"

We should (need to) implement the "CONVERT" option for "XA RECOVER" command, as is done in MySQL 5.7.

"XID values may contain nonprintable characters. As of MySQL 5.7.5, XA RECOVER permits an optional CONVERT XID clause so that clients can request XID values in hexadecimal."

https://dev.mysql.com/doc/refman/5.7/en/xa-statements.html

However, there is an existing MySQL bug about that specific implementation:

https://bugs.mysql.com/bug.php?id=87526

So we should address that bug as well when this is implemented, as one needs a usable Xid value for some of the XA commands, and this is not always obtainable from elsewhere.