Added last_communication timestamp to the adapter proto.

For VOL-2207.  Please consider these related patchsets together:
https://gerrit.opencord.org/#/q/VOL-2207

Change-Id: I75b82143db61d3bb49eb354c6b9e3203d8d6f16a
diff --git a/protos/voltha_protos/adapter.proto b/protos/voltha_protos/adapter.proto
index f5b2aa6..6cb2346 100644
--- a/protos/voltha_protos/adapter.proto
+++ b/protos/voltha_protos/adapter.proto
@@ -9,6 +9,7 @@
 import "google/protobuf/any.proto";
 import "voltha_protos/common.proto";
 import "voltha_protos/meta.proto";
+import "google/protobuf/timestamp.proto";
 
 
 message AdapterConfig {
@@ -39,6 +40,8 @@
 
     repeated string logical_device_ids = 4;  // Logical devices "owned"
 
+    // timestamp when the adapter last sent a message to the core
+    google.protobuf.Timestamp last_communication = 5;
 }
 
 message Adapters {