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
Separate Metrics struct from Master class.

Review: https://reviews.apache.org/r/27744/
Introduced mutex configure check in libprocess.

Review: https://reviews.apache.org/r/27350
Fixed the bug in validating duplicated task ID and added a test.

Review: https://reviews.apache.org/r/27903
Replaced Timer::create/cancel with Clock::timer/cancel.

Review: https://reviews.apache.org/r/27496
Updates to Mesos for Timer::create/cancel -> Clock::timer/cancel.

Review: https://reviews.apache.org/r/27497
Abstract clock internals from ProcessManager::settle.

Review: https://reviews.apache.org/r/27498
Introduced a callback for timer expiration.

This enables seperating the Clock specific functionality from the
ProcessManager functionality so that the Clock implementation can
ultimately be completely isolated.

Review: https://reviews.apache.org/r/27499
Removed redundant synchronization and conditional check.

Review: https://reviews.apache.org/r/27500
Simplified redundant Clock::order/update usage.

Review: https://reviews.apache.org/r/27501
Introduced a libev async watcher specifically for updating timers.

Review: https://reviews.apache.org/r/27502
Moved Clock implementation into clock.cpp.

Review: https://reviews.apache.org/r/27503
Started moving libev specific functionality out of process.cpp.

Review: https://reviews.apache.org/r/27504
Moved event loop specific polling into poll.cpp.

Review: https://reviews.apache.org/r/27505