grpc plugin: Create a "Dispatch" service and use streaming RPCs.

grpc plugin: Create a "Dispatch" service and use streaming RPCs.

This patch, unfortunately, does two things:
* It breaks out the DispatchValues() call into a separate service. The
  intention of this is to make it easier to implement only part of the
  API, namely to not implement querying of values, which may not be
  useful for stateless proxies.
* It turns the DispatchValues() call into a (client side) streaming RPC.
  That means that a client can send many ValueLists to the server in one
  streaming request, rather than many self-contained requests.

This code is heavily influenced by test code of the "protobuf-qml"
project, which is MIT licensed.