Support ObserveOn And Known Schedulers

[RxDave]

Currently, the ObserveOn operator can only be used with serializable IScheduler implementations; however, this is not the typically intended usage of ObserveOn. Rather, its usage implies that the server must perform scheduling using the specified scheduler, which is identified via a static property such as NewThreadScheduler.Default.

The client must rewrite queries containing constant scheduler references by replacing them with references to their corresponding static properties.