Bug 2158: Handle out-of-order inventory notification.

MD-SAL notifications are delivered on multiple threads.
So VTN Manager should not expect order of inventory notifications.

Change-Id: I9a23203b085ab114b8de11c6584903ab3850755e
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>
(cherry picked from commit 3192eba4c145930a1f54950966b9b451ebb12b51)