Fix contrail-collector back pressure mechanism

contrail-collector SM queue back presssure mechanism was not
working since the SM drop level is initialized to INVALID and
even the water marks levels are INVALID and hence the defer/undefer
callbacks are not called.

Change-Id: Ieaf65d3d77356b9ac471e9479a1fb743c16fe6a1
Closes-Bug: #1528380

library/cpp/sandesh_state_machine.cc

@@ -781,9 +781,10 @@ void SandeshStateMachine::SetSandeshMessageDropLevel(size_t queue_count,
             Sandesh::LevelToString(level) << "], SM QUEUE COUNT: " <<
             queue_count);
         message_drop_level_ = level;
-        if (!cb.empty()) {
-            cb();
-        }
+    }
+    // Always invoke the callback
+    if (!cb.empty()) {
+        cb();
     }
 }