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
Updated Mesos codebase to set the framework principal in FrameworkInfo.

Review: https://reviews.apache.org/r/22315
MESOS-390 Fetcher supports file://// URIs