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/LinkEvent.proto b/incubator/protobuf/src/main/proto/LinkEvent.proto
new file mode 100644
index 0000000..fdd6580
--- /dev/null
+++ b/incubator/protobuf/src/main/proto/LinkEvent.proto
@@ -0,0 +1,19 @@
+syntax = "proto3";
+option java_package = "org.onosproject.grpc.net";
+
+package LinkEvent;
+
+import "Link.proto";
+
+// Corresponds to org.onosproject.net.link.LinkEvent.
+message LinkNotification {
+    LinkEventType linkEventType = 2;
+    Link.LinkCore link = 3;
+}
+
+// Link Event Types
+enum LinkEventType {
+    LINK_ADDED = 0;
+    LINK_UPDATED = 1;
+    LINK_REMOVED = 2;
+}