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/core/net/src/main/java/org/onosproject/net/pi/impl/PiPipeconfWatchdogManager.java b/core/net/src/main/java/org/onosproject/net/pi/impl/PiPipeconfWatchdogManager.java
index 0503d9a..e76d673 100644
--- a/core/net/src/main/java/org/onosproject/net/pi/impl/PiPipeconfWatchdogManager.java
+++ b/core/net/src/main/java/org/onosproject/net/pi/impl/PiPipeconfWatchdogManager.java
@@ -241,7 +241,7 @@
         log.debug("Starting watchdog task for {} ({})", device.id(), pipeconf.id());
         final PiPipelineProgrammable pipelineProg = device.as(PiPipelineProgrammable.class);
         final DeviceHandshaker handshaker = device.as(DeviceHandshaker.class);
-        if (!handshaker.isConnected()) {
+        if (!handshaker.hasConnection()) {
             return false;
         }
         if (Futures.getUnchecked(pipelineProg.isPipeconfSet(pipeconf))) {