Fixed a bug in the status update manager to properly handle duplicate
ACK of an old update when waiting for the ACK of a new update.

Review: https://reviews.apache.org/r/10596

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1469118 13f79535-47bb-0310-9956-ffa450edef68