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.