Kafka Integration Application (Fix Javadoc errors + review comments)

1. Refactored the application into two java projects api and app as per convention
2. Deleted the onos-app-gpb project. The proto files are consolidated in the
   incubator-protobuf project as per suggestions.
3. Some code to translate ONOS Event pojo messages to GPB format.
4. Implementation of Subscribe and Unsubscribe APIs.
5. Minor changes due to review comments from 9212 and 9053
6. Refactored the proto fileso that its a 1:1 mapping between the core type to proto message.

Change-Id: I2bcc0de96150f838ccfe9e49293fe61d94062628
diff --git a/incubator/protobuf/src/main/proto/Link.proto b/incubator/protobuf/src/main/proto/Link.proto
index 45b9e90..b5fcbfc 100644
--- a/incubator/protobuf/src/main/proto/Link.proto
+++ b/incubator/protobuf/src/main/proto/Link.proto
@@ -38,6 +38,20 @@
   string port_number = 2;
 }
 
+enum LinkState {
+    ACTIVE = 0;
+    INACTIVE = 1;
+}
+
+// Corresponds to org.onosproject.net.Link.
+message LinkCore {
+  LinkState state = 1;
+  ConnectPoint src = 2;
+  ConnectPoint dst = 3;
+  LinkType type = 4;
+  map<string, string> annotations = 5;
+}
+
 message LinkDescription {
   ConnectPoint src = 1;
   ConnectPoint dst = 2;