Fix ValidateDateTime: check day instead month

Java caliper counter

Add python benchmark

Found with PVS-Studio static analyser, see 
https://www.viva64.com/en/b/0550/

Merge 3.5.x branch into master

whitelisting aix platform as it has sched_yield

Add caliper supported to java benchmark

remove nullptr

Fix uploading binary wheel.

Add tests to cover all the common special casing in the runtime code to
ensure things come out correctly.

The script now takes two prompt for user name and password. One single
input redirection no longer works.

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