Integrated internal changes from Google

Add go benchmark

Allow null value in JSON representation of ListValue

We need to use a new enough version to ensure that constexpr is
available.

Pre-5.1.0 versions of GCC do not support these particular type traits
(see https://github.com/google/protobuf/issues/417).

Cat the test-suite.log on errors for presubmits

Fix spelling error of __GNUC_MINOR__

Objective-C: Check return value on write of raw pointer

Remove 64MB memory limit when deserializing messages in C#

Support using MSVC intrinsics in Log2FloorNonZero

We no longer need this, now that we have finished the switch to C++11
and are using std::unique_ptr.

Revert "Removed mention of Buffer in byteSourceToUint8Array"

Ruby: Fix scope resolution for MessageExts

Output *_pb2_grpc.py files when use_grpc_plugin=True

Replace ancient m4/acx_pthread.m4 with m4/ax_pthread.m4

Start outputting missing *_pb2_grpc.py files when  use_grpc_plugin is set to True for py_proto_library.

This reverts commit f00e06c9.

Although Node Buffers are not covered by the tests, it seems that there
are users relying on them, so we need to add back in support for them.

This fixes issue #4359.

Revert "Move `compiler/plugin.pb.cc` to libprotobuf with the other WKT sources."

Replace //:protoc and similar default macro arguments with

@com_google_protobuf prefixed versions. This allows them to work in 3rd party
repositories.

Fix a bad visibility rule. :hidden does not exist, but :private does.