Update CHANGELOG for 0.21.0-rc3
Perform task validation after authorization is done.

Review: https://reviews.apache.org/r/27769
Moved all resources validation logic to ResourceUsageChecker.

Review: https://reviews.apache.org/r/27843/
Added c++11 initializer_list check to configure.

Review: https://reviews.apache.org/r/27945
Added a unit test for path::join.

Review: https://reviews.apache.org/r/27605
Added new <stout/path> test to the libprocess Makefile.

Review: https://reviews.apache.org/r/27946
Destroy Docker containers with stop instead of kill.

Ensure docker calls stop if a docker_stop_timeout is provided.
Added the flag docker_stop_timeout that defaults to 0, if the
timeout is 0 the a docker kill will be run, otherwise a docker stop
with the timeout in seconds is used.

Review: https://reviews.apache.org/r/26736