Implemented non-child process monitoring in reaper.

Review: https://reviews.apache.org/r/8570

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1455813 13f79535-47bb-0310-9956-ffa450edef68
Added os::islink and substituted stat() with
lstat() in os.hpp.

Review: https://reviews.apache.org/r/8681

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1455814 13f79535-47bb-0310-9956-ffa450edef68
Recovered slave state.

Review: https://reviews.apache.org/r/8680

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1455815 13f79535-47bb-0310-9956-ffa450edef68
Recovered status update manager.

Review: https://reviews.apache.org/r/8736

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1455816 13f79535-47bb-0310-9956-ffa450edef68