Use ProducerScope for server streaming calls

[rouzwawi]

This switches the signature on server streaming handlers to be executed within a ProducerScope<TReply>.

-  override suspend fun greetServerStream(request: TRequest) = produce<TReply> { ... }
+  override suspend fun ProducerScope<TReply>.greetServerStream(request: TRequest) { ... }

It also makes exception propagation more consistent across the various rpc types by always following these rules:

• Exceptions thrown from rpc handlers will either:
  • complete the request and send a status to the caller, if they are instance of StatusException or StatusRuntimeException
  • complete the request with a status UNKNOWN or INTERNAL, and propagated to the CoroutineScope that is used in the service constructor