More consistent hooks for scheduler plugins.  This is a rework of #1514.

* Deprecated RxJavaDefaultSchedulers and RxJavaDefaultSchedulersDefault (I think these names are confusing)
* Added RxJavaSchedulersHook
* Ability to override entire schedulers from scratch
* Ability to decorate Action0 before submission to Scheduler

The main change from #1514 is the decoration of Action0 instead of Runnable.  Action0 is an improvement as all Schedulers must handle these, whereas not all Schedulers handle Runnables.