Add an rpc option for fail quick for a non existing service

[igalshilman]

Sometimes it is useful to know that you are rpcing a service that does not exists.
This happens for a generic dispatchers use cases.
Let us add an explicit rpc option to fail fast for a non existing service.

This of course comes with the warning, that this might give false positives in case of a service discovery happening in parallel to an rpc. (metadata propagation, speed of light 🔦 )