Introduce a new decorator lymph.rpc2

(Spec url when merge will be include here)
Is it just the name or you think that it should be the same decorator ? b/c I don't think that it's a good idea to use the same decorator, since we should make sure that behaviour doesn't change dramatically which is not something that I can promise !

I am already dropping support of ack in the rpc2() since it's ambiguous, and with remote Exception catching which can be done with the explicit raises, this mean a new RPC protocol ... so it make sense to have a new decorator for it.

If we agree that it should be a new rpc decorator I can start a thread of names proposals ...

Thought ?