Down integrate to Github

Signed-off-by: Mickaël Schoentgen <contact@tiger-222.fr>

The bazel-bin/ and bazel-genfiles/ directories are going to be merged
soon. That change was causing some test failures on Windows, and I
believe it is because the logic for locating test data files was getting
confused and mistakenly looking for them under bazel-bin/. This commit
updates that logic to look for a more specific file (descriptor.cc)
which does not appear under any Bazel-related directories.

It sounds like the more official solution is to use the Bazel runfiles
library (i.e. //tools/cpp/runfiles). However, I decided not to do that
because we currently still support multiple build systems, and if we
used a separate solution for Bazel then I suspect we would need even
more #ifdefs in the code to handle the different systems.

Undelete python/google/protobuf/pyext/proto2_api_test.proto that was accidentally removed from file_list

* add check for reserved keywords in extensions

* add all reserved keywords

* use string without std::

* add test and update name in lite version

* test updates

also add useful Span-based methods for ByteString

Revert "Updating Iterators to be compatible with C++17 in MSVC"

Update CONTRIBUTING.md to recommend applying the appropriate labels

Make the C# benchmarks non-packable

C# testing improvements

* add terminating character

* adjust name length input

* add test cases

* correct script name

* Misc fixes

* add new script to EXTRA_DIST list

For reasons I don't follow, when passing CXXFLAGS to configure, the build
ends up breaking because something along the way fails to detect C++11
support within the third_party/googletest part of the build.

* Bump target frameworks from netcoreapp1.0 to netcoreapp2.2.
Move global.json up to root of repo, change SDK ver to 2.2.100
Change .net core sdk in dockerfile for kokoro to ver 2.2.100

* Re-add curl install

* Change all exe target to 2.1

* Fix incorrect versions in global.json and Dockerfile

* Downgrade version to 2.1 to match exe targets

* introduce separate testing Dockerfile for C#

* revert changes to the shared Dockerfile

* use netcoreapp2.1 for C# conformance tests

* use language specific dockerfile for testing C#

* Edit compatibility tests script to use parameters instead of file copies

* install dotnet SDK on windows before running the tests

* update csharp_EXTRA_DIST

* Improve C# serialization performance of repeated fields for primitives.

* Changes based on feedback.

* Change compatibility tests to chec float, bool and double are fixed

* Changes based on feedback.

* In the compute methods use the newly created constants

Recently dependency to error_prone_annotations was added to the code,
but only Maven build tool chain was updated.

Closes #5795.

* Fix ruby continuous tests

* Fix compatibility test

* Add link to continuous test badges

* Add badge for dist_install

* Add a new test for verifying that the dist package can be installed.

* Some fixes to make the code work in google3.

* Removed plugin.h.

* Some more fixes to be namespace-independent.

* More fixes for namespace independence.

* A few final fixes.

* Another fix (hide ToUpper from Copybara).

* Fix for charp_unittest.

This change introduces a CriticalSectionLock class as a lightweight replacement for std::mutex on Windows platforms. std::mutex does not work on Windows XP SP2 with the latest VC++ libraries, because it utilizes the Concurrency Runtime that uses ::GetLogicalProcessorInformation which is only supported on Windows XP SP3 and above.