* Down-integrate internal changes

* Update generated code for php, objc and csharp.

* Add missing dependency in conformance_php

Replace update_version shell script with a python script, which is much better at handling XML rewrite. Update the PHP version too. As part of rewrite, some XML file format will change a little, but the semantics is still the same so it should be okay.

Use docker for csharp, java_compatibility, and python_compatibility. Remove prepare_build_linux_rc that is no longer needed.

1. Fix C++ tests.
  * Add missing files to Makefile.am and fix distcheck in tests.sh
  * Remove BUILT_SOURCES from conformance/Makefile.am.
  * Add some missing override keyword.
  * Add a type cast to int64 because our StrCat() in stubs can't handle size_t.
2. Fix Java tests.
  * Add missing test dependency on guava in pom.xml.
  * Include newly referenced test data in test resources.
  * Manually fix map_lite_test.proto which is overwritten because it's mapped
    from map_test.proto in google3.
  * Add back "optimize_for = LITE_RUNTIME" which is still needed to keep the
    opensource test passing as it's still running lite tests.
  * Add a type cast in newBuilder() because without it the code doesn't compile
    with openjdk javac 1.8 (the compiler can't figure if it's the right type
    due to complex generic typing).
3. Fix Python tests.
  * Remove/replace references to <hash_map>.
  * Suppress more warnings in setup.py.
  * Replace incorrect header inclusion for google/protobuf/proto_api.h.
  * Add strings::EndsWith to google/protobuf/stubs/strutil.h because it's
    referenced in the updated python C extension code.
  * Replace proto2 with google::protobuf. The proto2 name is leaked to
    opensource because we removed the subsitition rule for proto2 namespace
    but only fixed C++ source code and forgot to update python C extension code.

* Ignore unknown enum received in json when ignoreUnknownFields flag is set.

* Store the class object itself

* Add test for backwards compatibility

* Rename and add test

* Add comments

* Fix setup.py for windows build.

* Bump version number to 3.5.2

* Cat the test-suite.log on errors for presubits

Fixes #4220.