Update CHANGELOG for 0.21.0-rc3

Update CHANGELOG for 0.21.0-rc3

Add to 0.21.0 CHANGELOG.

CHANGELOG and docs/upgrades.md for 0.21.0.

Review: https://reviews.apache.org/r/27633

Updated CHANGELOG for 0.20.1-rc3.

Updated CHANGELOG for 0.20.1-rc3.

Updated configure.ac to 0.20.1.

*** MODIFIED for 0.20.1-rc1 *** Added CHANGELOG for 0.20.1.

Fixed a bug in docker/docker.cpp that causes unresolved symbols on OSX.

Review: https://reviews.apache.org/r/24825

Fixed a bug in docker/docker.cpp that causes unresolved symbols on OSX.

Review: https://reviews.apache.org/r/24825