VOL-1334 : Fixed concurrency issues

- Semaphores were added at the different layers of the model
- Made the proxy interfaces more robust
- Eliminated problems while retrieving latest data in concurrent mode

Change-Id: I7854105d7effa10e5cb704f5d9917569ab184f84
diff --git a/db/model/callback_type.go b/db/model/callback_type.go
index 8f55688..b530dee 100644
--- a/db/model/callback_type.go
+++ b/db/model/callback_type.go
@@ -13,10 +13,13 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package model
 
+// CallbackType is an enumerated value to express when a callback should be executed
 type CallbackType uint8
 
+// Enumerated list of callback types
 const (
 	GET CallbackType = iota
 	PRE_UPDATE