Added an abstraction for launching operations in a subprocess.

Review: https://reviews.apache.org/r/22224
Added setns utilities to stout.

Review: https://reviews.apache.org/r/22160
Added virtual destructor to Operation base class.

Review: https://reviews.apache.org/r/22311
Added terminal action to routing lib.

Review: https://reviews.apache.org/r/22310
Allowed net::IP to be created from a dot decimal string.

Review: https://reviews.apache.org/r/22294
Used the new net::IP interfaces.

Review: https://reviews.apache.org/r/22295
Temporarily disabled flaky AllocatorZookeeperTests due to MESOS-1337,
MESOS-1341 and MESOS-1436.

Review: https://reviews.apache.org/r/22312
Fixed a typo in setns.hpp

Review: https://reviews.apache.org/r/22319
Increased failover timeout for framework in
AllocatorTest/SchedulerFailover test.

Review: https://reviews.apache.org/r/22330
Added an equal operator for IntervalSet.

Review: https://reviews.apache.org/r/22331
Added a hash function for PortRange.

Review: https://reviews.apache.org/r/22333
Removed deprecated 'offer_id' from LaunchTasksMessage.

Review: https://reviews.apache.org/r/22266
Updated reconciliation to send statuses even if state matches.

Review: https://reviews.apache.org/r/22267
Added "implicit" reconciliation.

Review: https://reviews.apache.org/r/22268
Initial import for health check
Fixed Protobuf to JSON serialization of optional fields with default values.

Previously, only optional fields which where explicitly set would be serialized regardless whether default values was present.
This patch adds those fields explicitly.

Review: https://reviews.apache.org/r/21246
Cleaned up JSON work
Snapshot of current health check program.
Add task health check