Galera depends on deprecated option innodb_locks_unsafe_for_binlog

The option innodb_locks_unsafe_for_binlog was deprecated already in MariaDB Server 10.0.0. Yet, Galera documentation recommends setting this option.

In MariaDB Server 10.5, MDEV-19544 will finally remove the deprecated option.

To provide a smoother upgrade path, I think that we should stop recommending or setting innodb_locks_unsafe_for_binlog=1 already in the 10.4 GA version, and if possible, in earlier versions as well.

If the correct operation of Galera currently depends on this setting, then we should extend the WSREP patches in InnoDB so that when Galera is use, the gap locks will continue to be skipped. I think that it would be a must to have these changes in 10.4, and it would be nice-to-have in earlier major versions.