Fixed zookeeper detector by ignoring ZNONODE error in detectMaster()
when it happens after getting child znodes but before getting their
data.

Review: https://reviews.apache.org/r/10105

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1461915 13f79535-47bb-0310-9956-ffa450edef68
Fixed slave to not garbage collect old slave directories based on
their modification time.

Review: https://reviews.apache.org/r/10162

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1461938 13f79535-47bb-0310-9956-ffa450edef68