Add changelog for 3.9.x

This reverts commit 7f84a94.
This is just temporary. Eventually, we still want to roll forward this
change. Some users are complaining they need more time to clean up their
code.

File io_win32.h is not in directory google\protobuf\stubs under directory google\protobuf\io

Forgot to set it previously.

https://github.com/matthew-brett/multibuild/pull/240

* Fix binary compatibility in FieldCodec factory messages

* Make default value parameter for current factories required

* Route old methods through default value overloads

…ange

* Rename internal ExtensionSet.GetValue to TryGetValue

* Disable javadoc error

Actual fixes of the javadoc will be followed up

* Remove duplicated configuration

* Update javadoc plugin version

…6433)

I'm not exactly sure why, but this fixes the failing Bazel presubmit
test. Using the most recent version seems like a good idea anyway so
that we can make sure we're compatible with any new Bazel changes.

Backport review comments for C# proto2 features (for 3.9.x)

generate_descriptor will handle that