Fix issues discovered in up-integration.

Add status badge for bazel build.

Fix bazel test.

* Fixes https://github.com/google/protobuf/issues/5031
* Fixes https://github.com/google/protobuf/issues/5037

Down-integrate proto2 namespace fix.

Fix json parsing of wrapper values

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