MESOS-1392: MasterDetector now returns a None when it cannot read the content of the ZNode it has detected.

Review: https://reviews.apache.org/r/25663
Modify Docker::pull to call inspect after pull.

Review: https://reviews.apache.org/r/25758
Updated CHANGELOG for 0.20.1-rc3.
Replace os::Release with Version.

Review: https://reviews.apache.org/r/25755
Updated callers to os::release in mesos.

Review: https://reviews.apache.org/r/25780
Fixed a signed comparision issue in Version.
Fixed example frameworks to launch tasks based on offered resources.

Review: https://reviews.apache.org/r/25801
Fixed example frameworks to fail fast on an unexpected status udpate.

Review: https://reviews.apache.org/r/25805
Hold on to unacknowledged terminal tasks in the Master.

Review: https://reviews.apache.org/r/25567
Added tests for terminal unacknowledged tasks in the Master.

Review: https://reviews.apache.org/r/25568
Fixed a segfault regression in the master.

Review: https://reviews.apache.org/r/25810
Updated a comment regarding GCC 4.7 in configure.ac and some syntax
formatting.

Review: https://reviews.apache.org/r/25218
Ignore terminal task resources in the Master's Framework struct.

Review: https://reviews.apache.org/r/25844
Fixed an executor accounting bug in framework re-registration.

Review: https://reviews.apache.org/r/25843
Fixed an issue with colored output in post-reviews.py.

Review: https://reviews.apache.org/r/25819
Refactored Node class in libprocess.

Moved class Node out of process.cpp and into its own header as a part
of refactoring process.cpp.

Review: https://reviews.apache.org/r/25847