Fixed slave to properly handle terminated tasks that have pending
updates.

Review: https://reviews.apache.org/r/13187
Improved StatusUpdateManager's API by flattening the return types.

Review: https://reviews.apache.org/r/13196
Fixed slave to remove a queued task when the task is terminated.

Review: https://reviews.apache.org/r/13207
Refactored slave recovery so that slave recovers its state before
the isolator and status update manager.

Review: https://reviews.apache.org/r/13216
Fixed cgroups isolator to not try to destroy already destroyed
cgroup during recovery.

Review: https://reviews.apache.org/r/13224