Galera Cluster failed when two transactions that were executed
serially on a master node were incorrectly executed in parallel on a
slave node. The slave node detected MDL BF-BF conflict and quit the
cluster.

The problem was caused by OPTIMIZE TABLE on a table that is the child
table of a foreign key constraint. If UPDATE was performed on the
parent table of the foreign key constraint while OPTIMIZE TABLE was
running on the child table, the master node would run the transactions
serially, but a slave node might run them in parallel resulting in MDL
BF-BF conflict. The problem is fixed by adding foreign key constraint
to the replicated write set preventing parallel apply of the
transactions on the slave node.

Removed debug messages.