Jcstress for Concurrency related tests

[mipo256]

I was just debugging an issue in the concurrency-related integration tests in the JdbcRepositoryConcurrencyIntegrationTests, and the setup for concurrency is a bit clumsy, to put friendly. I think, we can introduce the jcstress JDK library for that.

What are your thoughts on this @schauder @mp911de? That would unify the way the concurrency tests are set up.

[mp911de]

Tests mentioned above provide a rather explicit way of guarding and latching. Someone not familiar with jcstress has a steep learning curve and that sort of tests expect a high throughput to catch bugs that are likely in high-concurrency scenarios. For the time being, I suggest not introducing yet another tool. If you feel, that JdbcRepositoryConcurrencyIntegrationTests can be optimized for readability, feel free to submit a PR so we can discuss whether we share the same understanding of better readability.

[mp911de]

We discussed this item with the team and concluded that we want to keep things as they are. Both, JMH and JCStress work very similar in terms of running them. JMH already requires quite some dance and we don't want to open up another area of work just because of the style. We admittedly don't have that many of concurrency tests and we're happy with the way they look. They also do not cause so many issues that we are inclined to fix something.