Reduce contention on commit order monitor

The optimization is available at https://github.com/percona/percona-xtradb-cluster/commit/c01c75da8ec67197e5b2f86edbdd9541df717faa , but massive changes are required due to differences between PXC and MariaDB code affected by the patch. The value of the optimization is described in https://www.percona.com/blog/2017/04/19/how-we-made-percona-xtradb-cluster-scale/ and https://www.percona.com/blog/2017/04/19/performance-improvements-percona-xtradb-cluster-5-7-17/