Add jcstress concurrency tests

[l0rinc]

Now that jcstress is finally available on Maven, we should create some stress tests.

Some contenders with mutable inner state that should definitely be validated:

• javaslang.Lazy#computeValue
• javaslang.λ.Memoized#of
• javaslang.concurrent.Future
  • find, await, cancel, complete, onComplete, isEmpty, run, tryComplete

[danieldietrich]

concurrent.Future is out of scope, it is removed in Vavr 1.0