Add TASK_ERROR to CHANGELOG for 0.22.0

Review: https://reviews.apache.org/r/27930
Added module loading to scheduler driver.

Review: https://reviews.apache.org/r/27806
Integrated CRAM-MD5 Test Authenticatee module into scheduler.

Review: https://reviews.apache.org/r/27812
Updated upgrades.md and CHANGELOG for changes on AuthenticationStartMessage.

Review: https://reviews.apache.org/r/27675
Fixed scheduler driver to not acknowledge status update when stop() is called.

Review: https://reviews.apache.org/r/27896
Fixed a bad logging message in libprocess initialization.

Review: https://reviews.apache.org/r/27932
Updated CRAMMD5Authentication tests to additionally run against test authenticator module.

Review: https://reviews.apache.org/r/27619
Updated CRAMMD5Authentication tests to additionally run against test authenticatee module.

CRAMMD5Authentication tests now cover all default and modularized
authenticator/authenticatee combinations.
Also adds a test for missing credential for the authenticatee.

Review: https://reviews.apache.org/r/27651