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/Port.proto b/incubator/protobuf/src/main/proto/Port.proto
index 0d9ed2e..a3fd34d 100644
--- a/incubator/protobuf/src/main/proto/Port.proto
+++ b/incubator/protobuf/src/main/proto/Port.proto
@@ -31,6 +31,15 @@
   map<string, string> annotations = 8;
 }
 
+// Corresponds to org.onosproject.net.Port.
+message PortCore {
+  string port_number = 1;
+  bool is_enabled = 2;
+  PortType type = 3;
+  int64 port_speed = 4;
+  map<string, string> annotations = 5;
+}
+
 message PortStatistics {
   int32 port = 1;
   int64 packets_received = 2;