Perper Functions: change the notification handling logic to support multiple runtimes

[branimirangelov]

Decouple the notification handling of the function runtime from the notification handling of the stream updates. This will enable the non-dotnet language workers to handle the notifications by their own API.

[branimirangelov]

Investigate GRPC multiplexing of streams

[branimirangelov]

Partly implemented as adding JSON serialization support in PerperValueBinder. This will allow language runtimes to establish own GRPC connection. The actual language binding implementation is planned for 0.6