This allows us to filter `IEndpointSelectorPolicy` instance based on
whether the apply to a given candidate set. This should allow us to
remove some HAXXX from MVC.
The idea here is the ESP becomes much more pay-for-play if you can
statically eliminate many of the cases where it would usually no op.
These changes are based on our discussion earlier this week. Adding
async, and making it possible to short circuit, which should be better
aligned with the requirments of versioning.
Ryan Nowak
James Newton-King