Testing and improving 'cluster conflict resolving' with 10.4 and later

There are a number of bug reports of cluster wide conflict resolving related crashes or hangs. To troubleshoot these, it would be good to run generic cluster conflict tests, just for easier reproducing of the issue(s).

Run high intensity write-write conflict load in the cluster, with varying SQL access patterns.
Also, some mtr tests exercising high priority victim aborting, appear to be disabled in 10.4. Enable these and fix the test if it appears to be non-deterministic.

Analyze and troubleshoot issues surfacing from the testing.