Add support for enabling/disabling ports for gNMI devices

This change also includes:
- Refactoring of gNMI protocol+driver to take advantage of the recent
changes to the gRPC protocol subsystem (e.g. no more locking, start RPC
with timeouts, etc.).
- Fixed Stratum driver to work after GeneralDeviceProvider refactoring
- Updated bmv2.py to generate ChassisConfig for stratum_bmv2
- Fixed portstate command to use the same port name as in the store

Change-Id: I0dad3bc73e4b6d907b5cf6b7b9a2852943226be7
diff --git a/protocols/grpc/api/src/main/java/org/onosproject/grpc/api/GrpcClient.java b/protocols/grpc/api/src/main/java/org/onosproject/grpc/api/GrpcClient.java
index d529529..7957006 100644
--- a/protocols/grpc/api/src/main/java/org/onosproject/grpc/api/GrpcClient.java
+++ b/protocols/grpc/api/src/main/java/org/onosproject/grpc/api/GrpcClient.java
@@ -28,11 +28,8 @@
 
     /**
      * Shutdowns the client by terminating any active RPC.
-     *
-     * @return a completable future to signal the completion of the shutdown
-     * procedure
      */
-    CompletableFuture<Void> shutdown();
+    void shutdown();
 
     /**
      * This method provides a coarse modelling of gRPC channel {@link