Fixed Mutlimap and multihashmap by removing the ordering requirement
on values.

Review: https://reviews.apache.org/r/10205

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1466157 13f79535-47bb-0310-9956-ffa450edef68
Added keys() to Multimap and multihashmap.

Review: https://reviews.apache.org/r/10206

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1466158 13f79535-47bb-0310-9956-ffa450edef68
Added 'unschedule' operation to the GarbageCollector, changed schedule
to discard the future when unscheduled/rescheduled, added
GarbageCollector unit tests.

Review: https://reviews.apache.org/r/10028

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1466159 13f79535-47bb-0310-9956-ffa450edef68