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.

Add protobuf-all in post release

conflict

Support win32 long path for cross compiled build

Update changelog

Backport #3858 to 3.5.x branch

Fix Atomic32/AtomicWord on some platforms.

- Removed the redefination in the atomicword_compat.h. Now Atomic32 will
alwyas be intptr_t in 32bit platforms, the bridge is no longer needed.
- Change NaCl 64bit to use intptr_t instead of int32 for Atomic32,
otherwise AtmoicWord APIs will miss the definations.

After this change, the AtmoicWord will always be either Atomic32 or
Atmoic64 depending on the platforms.

Update version number to 3.5.0

* Fix php well known type conformance tests

* Properly generate code for test.proto

* Provide GPBMetadata files in c extensions for generated files to import.

* Remove unnecessary test

* Clean up code

* Add declaration for initOnce.

* Refactoring

If messages A and B have the same oneof case, which is a message
type, and we merge B into A, those sub-messages should be merged.

Fixes #3200.

Note that I haven't regenerated all the code, as some of the protos
have been changed, breaking generation.

update joda-time javadoc url

Note that the default behavior is still to throw an exception; you
need to opt into ignoring unknown fields.

Fixes #2838.

Suppress VS2017 compiler/linker warnings

* Reserve unknown in ruby

* Revert ruby tests. Wait for cpp impl for conformance test

* Add conformance test for preserving unknown

* Add unknown field conformance test to csharp failure list.

* Fix comments

* Fix comment

* Fix comments

* Fix typo

* Use stringsink_string directly

* Mark hd unused

* Remove unused encodeunknown_handlerfunc

Merge 3.4.x into master before cutting 3.5.x

To be clear that we set a new variable