UnitTest to assert thread safety of Executor Scheduler

- The Executor thread-pools hop threads when using observeOn so this tests that it is thread-safe when doing so.
- The test performs non-thread-safe mutations which should fail (or at least be non-deterministic) if memory visibility problems or concurrency is occurring.
Merge pull request #619 from benjchristensen/executorSchedulerConcurrencyTests

UnitTest to assert thread safety of Executor Scheduler