Allow sharing the same gRPC channel between clients

This change introduces a refactoring of the gRPC protocol subsystem that
allows the creation of a gRPC chanel independently of the client, while
allowing multiple clients to share the same channel (e.g. as in Stratum
where we use 3 clients).

Moreover, we refactor the P4RuntimeClient API to support multiple
P4Runtime-internal device ID using the same client. While before the
client was associated to one of such ID.

Finally, we provide an abstract implementation for gRPC-based driver
behaviors, reducing code duplication in P4Runtime, gNMI and gNOI drivers.

Change-Id: I1a46352bbbef1e0d24042f169ae8ba580202944f
diff --git a/drivers/stratum/BUILD b/drivers/stratum/BUILD
index 1d19c3c..a87c97e 100644
--- a/drivers/stratum/BUILD
+++ b/drivers/stratum/BUILD
@@ -4,6 +4,8 @@
     "//drivers/gnmi:onos-drivers-gnmi",
     "//drivers/gnoi:onos-drivers-gnoi",
     "//pipelines/basic:onos-pipelines-basic",
+    "//protocols/grpc/api:onos-protocols-grpc-api",
+    "//protocols/grpc/utils:onos-protocols-grpc-utils",
 ]
 
 osgi_jar(