Enable fire and forget rpc calls.

[F483]

Currently the protocol assumes that every request is to receive a response. Since the protocol is for UDP where lost packets are to be expected, users cannot rely on getting a response and may design there usage so that it is not required.

For this case it can be of an advantage to enable fire and forget calls that do not require a response and thus reduce traffic. To enable this an additional datagram prefix could be used.

PS: I plan eventually adding end to end encryption to rpcudp, or forking if it makes more sense. Since this would break compatibility. I'm adding additional issues that also would require a break. This should keep everyone on the same page as to a possible future direction.

[bmuller]

I'd accept a PR for this.