Fix wrong usage of ISchedulerLongRunning in ObserveOn

[akarnokd]

The optimized ObserveOn was using the ISchedulerLongRunning scheduler incorrectly. This type of scheduler is meant to be kept alive by possibly blocking the backing thread as long as the sequence is still active.

Additional changes:

• Removed the ISchedulerLongRunning cases from the ObserveOnObserverNew implementation.
• The ObserveOnObserverNew no longer supports error delaying (Based on unit tests, it was expected only from ISchedulerLongRunning schedulers anyway).
• Created a new ObserveOnObserverLongRunning dedicated to working with ISchedulerLongRunning.
• The choice of which implementation to use has been moved to assembly time inside the ObserveOn extension method.
• Added unit test to verify there is no thread hopping with the TaskPoolScheduler.
• Added unit test to verify applying DisableOptimizations to TaskPoolScheduler and calling AsLongRunning() no longer returns an ISchedulerLongRunning implementation.

Fixes #877