The previous part I,II commits of the MDEV-31949 branch implement
XA-commit,rollback commands (shortcut as XAC,XAR here and elsewhere)
processing in the Engine from inside binlog group commit, similarly
how it's done to the normal transaction's commit.  Under enabled
binlogging the execution path is now the same for normal and xa
transactions:

       MYSQL_BIN_LOG::run_commit_ordered():
       ...
       /* for each hton do */
         hton->{commit_ordered,commit_by_xid,rollback_by_xid}
       ...
       return rc;

The current commit implements flush-log-later policy of durable write
at commit for the user xa. Unlike the normal trx case, XA-rollback
also needs a similar addressing.