Added support for incompatible upgrade.

Review: https://reviews.apache.org/r/8763

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1455818 13f79535-47bb-0310-9956-ffa450edef68
Garbage collected task meta directories.

Review: https://reviews.apache.org/r/8837

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1455819 13f79535-47bb-0310-9956-ffa450edef68