Integrated internal changes from Google

Sync benchmark changes from internal

Add a check_version() for use in WORKSPACEs

Travis changed their default image:
https://blog.travis-ci.com/2017-11-21-xcode8-3-default-image-announce

So there is no need to set a specific image any more, and the non
apple language tests should be able to use the default images.

Merge 3.5.x branch into master

* Provide discardUnknownFields API in php implementation

* Provide discardUnknownFields API in php c extension.

[Java] Add a UTF-8 decoder that uses Unsafe to directly decode a byte buffer.

Adding Release_CompareAndSwap 64-bit variant

unwrap descriptor class before comparison of RepeatedField types

Added back in LIBPROTOC_EXPORT which was removed by mistake

Upgrading dotnet to 1.0.4

[PROTOBUF-3404] add --with-zlib=PATH to configure.ac script

Add _file_desc_by_toplevel_extension back

Add _file_desc_by_toplevel_extension back for compatibility issue with 3.4.1.

_file_desc_by_toplevel_extension is a temporary solution for FieldDescriptor.file.
FieldDescriptor.file was added in code gen. However, we need to keep it for some time for compatibility reason (with 3.4.1 only). May try to remove it after 2 years.

Remove unhelpful build warnings

Include .cc and .h files in Python source distribution

-Wno-writable-strings removes 230 "ISO C++11 does not allow conversion from
string literal to 'char *'" warnings from TensorFlow test / build output.
These happen because pyext/ sources pass string literals to Python C API data
structures, e.g. PyGetSetDef, which for some reason were designed to not have
the const qualifier.

Use the ivar for the -open, so hopefully it lines up with the -close
in dealloc and isn't seen as an unbalanced "open".