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