Added Future::then which takes result of defer.

Review: https://reviews.apache.org/r/9569

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1449438 13f79535-47bb-0310-9956-ffa450edef68
Simplified Future::then(_Bind) implementations.

Review: https://reviews.apache.org/r/9570

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1449439 13f79535-47bb-0310-9956-ffa450edef68
Renamed 'deferred' to 'Deferred'.

Review: https://reviews.apache.org/r/9571

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1449440 13f79535-47bb-0310-9956-ffa450edef68