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".

Stop having travis build the test matrix and make it explicit so extra
settings are only on the test that need the, this helps make the travis
UI a little cleaner (linux hosted stuff doesn't list an Xcode version),
as well as avoid unneeded requirements limiting the travis pool something
can run on.

Small improvements to objectivec/DevTools/full_mac_build.sh to avoid some
repeated scripting and hopefully make things more reliable work with Xcode
9.x.

Don't mark the iOS tests as able to fail, hopefully with the new settings
they are stable and won't give false signals.

Simplify the case for xcode versions and fixup xcode 9.[0-1].

Add runtime asserts (that can be disabled in release) that verify
that the types being get/set for messages using the C Api match
the type in the descriptor for the field being get/set.

Fix up warnings from Xcode 9.1

Add protobuf-all in post release

conflict

Here's the pull request that made it happen: https://github.com/google/protobuf/pull/2951

Fixes 32bit tests.

Allow one to omit building libprotoc and protoc binaries