generate_descriptor will handle that

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

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.

* Disable javadoc error

Actual fixes of the javadoc will be followed up

* Remove duplicated configuration

* Update javadoc plugin version

* Rename internal ExtensionSet.GetValue to TryGetValue

* Fix binary compatibility in FieldCodec factory messages

* Make default value parameter for current factories required

* Route old methods through default value overloads

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

Forgot to set it previously.

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

This reverts commit 7f84a943.
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.

Add changelog for 3.9.x

Merge master into 3.9.x

This should fix issue #6186. I also had to remove a couple unnecessary
const keywords in map_util.h because the no_warning_test was showing
errors about those.

Conflicts:
	java/lite/pom.xml
	java/util/pom.xml

3.8.x merge

This reverts commit 9a8ef05a.

Add extension number for protoc-gen-validate

Compatibility tests pull artifacts over HTTPS

Add `protobuf_java_lite` Bazel target

Otherwise, sandbox and remote execution will complain `google/protobuf/port_def.inc` doesn't exist.

* Make php message class final

Because proto messages are just data object, they should not be mocked.
Explicitly make them final to avoid accidental usage.

* Update generated descriptors

Removed all use of STLDelete*() in ObjC and C# Generators.

Include protobuf_deps.bzl in generated release archives

* php: Fixed php notices for unknown enum indices

* php: Fixed formatting of Duration

This fixes:
* Missing nanoseconds. The nanoseconds where divided as a float and
  implicitly converted to a string before being passed to bcadd.
  This can result in a float formatted in scientific/exponential notation,
  which bcmath doesn't understand.
* Durations are supposed to be formatted without trailing zeroes.

This fixes #5918.

We found that some code depended on the behavior of appending to the
string without clearing it first. The comments in json_util.h also make
it appear that the appending behavior is intentional, so I think we need
to stick with the old behavior.

This reverts commit 3cbe45a4.

Other languages will have this removed inside Google first.
These functions are subject to ADL and cause problems.

Update generated codes

Update GSON version to 3.8.5