Disable pip cache when testing uploaded packages

Otherwise, pip complains "cannot find a version satisfies the
requirement." I suspect it was due to caching issues. I usued to
manually add a `sleep 30` between the upload and the installation test
which is quite unstable.

Shard 64-bit Linux languages into different Kokoro jobs

Migrate Jenkins jobs to Kokoro

Update changelog for 3.5.1

Add support for Windows ARM64 build

Add support for Windows ARM64 build

Calling Keychecker before checking key in MessageMap

resolved issue 4032 and added a unit test

Cherrypick for csharp, including:

Fix string::back() usage in googletest.cc

Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

Replace C++11 only method std::map::at

Update version number to 3.5.1

Fixes #4060

`umask 0022` makes sure the created package will be accessible by all users.

Fix file permission for python package.

Avoid calling method from php extension directly

Create containing directory before generating well_known_types_embed.cc

`umask 0022` makes sure the created package will be accessible by all users.

Java: generate an annotation to suppress deprecation warnings