ExecutionContext story should be simplified by eliminating ExecutionContextFactory

The relationship between the concrete ExecutionContext class and its implemented ExecutionContextFactory interface is a bit confusing and non-intuitive. In particular, it's not really clear without understanding the implementation of ExecutionContext when to use the ExecutionContext constructor vs. the factory's create methods. The factory should be eliminated, and the relevant "create" methods in the context should be changed to "with" methods, mimicking the other existing, more readable pattern used by ExecutionContext to, for instance, create a context with a different namespace registry.