lib: grpc: rework RPC handlers improve code clarity

lib: grpc: rework RPC handlers improve code clarity

- split NewRpcState object into 2, a Unary and a Streaming variant, which
  then allows for the next.
- move all state machine details inside these new state objects
  - use a template arg to allow for Streaming state tracking object
    creation and deletion w/o requiring this in each specific RPC
    hander.
- Code is more rugged by design now.

Thanks to Rafael Zalamena <rzalamena@opensourcerouting.org> for the cleanup
ideas/motivation.

Signed-off-by: Christian Hopps <chopps@labn.net>