SET TRANSACTION ISOLATION LEVEL applies to the very next transaction on
the connection. zmDbDo's success Debug INSERT INTO Logs is a real
statement on the same $dbh; with database debug logging enabled, that
INSERT becomes the "next transaction" and silently consumes the
isolation directive. The intended READ COMMITTED then never applies to
the prune/resync/delete TX that follows.
Call $dbh->do directly for SET TRANSACTION in both Event::delete and
zmstats.pl, bypassing zmDbDo's logging. SET TRANSACTION can't deadlock
so zmDbDo's retry was no benefit here anyway.