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/branches/0.12.x@1462222 13f79535-47bb-0310-9956-ffa450edef68
*** MODIFIED FOR 0.12.1 *** Fixed slave to not garbage collect old
slave directories based on their modification time.

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


Conflicts:
	src/slave/slave.cpp

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/branches/0.12.x@1462223 13f79535-47bb-0310-9956-ffa450edef68