Add JS and Protobuf.js benchmark, fix js's reader.skipGroup

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.

Cleanup kokoro scripts.

Changes:
1. Remove stuff no longer needed. Lots of the heavy lifting were there
because we were running our own jenkins cluster and had to manage all
the test logs ourselves. Now they are useless.
2. Change "-j2" to "-j4" to speed up the test a little bit. Kokoro
machines have 4 logic CPUs according to their spec.

Fix broken test of const repeated field iterators

The old version had a NOP for loop, which doesn't make sense. I'm pretty
sure this was the original intention. Newer GCC's
-Wunused-but-set-variable flags the old version.

Fix js reader.js's skipGroup

Fix typo in FieldMaskTree.java warning

#4593 Replace all occurrences of "NULL" to nullptr in src/google/proto…

If you make up a module map for Objective C protocol buffers, the compiler will complain about missing a declaration for GPBUnknownFieldSet which is used in this file.

Ignore python .egg file to make git status clean #5004

Added support for building the aarch64 and ppcle64 protoc binaries with Kokoro

This was fairly straightforward using the existing build-protoc.sh
script. The only problem I ran into was that the x86 Docker builds
create output directories owned by root, which caused some permission
issues. Fortunately it was easy to get around that just by doing those
Docker builds last.

Merge 3.6.x branch into master

tests: fix link failure and stack overflow on Mingw w64

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

fix third_party/benchmark submodule init 

I have added the xsrpcj RPC implementation for java

I made a few small fixes to the documentation related to publishing
protoc artifacts:
- The target directory for Mac should be called osx instead of macos.
- There needs to be a directory for aarch_64.
- We need to avoid calling "mvn clean" inside the protoc-artifacts
  directory, since that will delete the contents of the target/
  subdirectory.

* Give a unique category to each test.

This change introduce a TestCategory enum to ConformanceRequest. Existing tests
are divided into three categories: binary format test, json format test and json
format (ignore unknown when parsing) test. For the previous two categories, there
is no change to existing testee programs. For tests with the last category, testee programs
should either enable ignoring unknown field during json parsing or skip the test.

* Fix python test

* Fix java

* Fix csharp

* Update document

* Update csharp generated code

* Add conformance test for php c back

php c extension has different result for conformance test for different
php version and architecture. Try to add conformance back for php 7.1 c extension first.

* Disable conformance test for c extension on 32-bit architecture
32-bit and 64-bit have different failing tests

* Fix typo