Merged 3.3.x branch to master

This target will be used by gRPC iOS bazel build system.

Fully qualify min as std::min in wire_format_lite.cc

Remove "using namespace std" from stubs/common.h

Remove unused constants.

This prevents the contents of the std namespace from being effectively
pulled into the top-level namespace in all translation units that
include common.h. I left in individual using statements for a few common
things like std::set and std::map, because it did not seem worth going
through the churn of updating the whole codebase to fix those right now.

When compiling with -Werror, -Wunused-const-variable the build fails due
to those two constants not being used.

Add a link to dart-lang/protobuf

Fix python3 issue.

descriptor.proto uses proto2 syntax, which is not ready for external
usage. However, some proto3 files import descriptor.proto and cannot be
used. In this PR, all references (We cheated by only removing
extensions, which is enough for now. User should avoid using messages
defined in descriptor.proto as field type.) to content in
descriptor.proto are removed from generated files. Those that import
descriptor.proto can be used like other proto files.

Add gogoprotobuf to third-party add-ons list

Fix the bug by sharing the code for generating class name for both
message and enum.

add a key to ctx.action dict to prevent protoc losing the default env

Add node-protoc-plugin to "Other Utilities"

Simplify the Element dtor invocation when freeing elements in InternalDeallocate

Tighten up class usage/checks.

- Ensure extensions resolution/wiring is happening directly on the
  messageClass (incase someone is doing odd things our out classes).
- Make the extension message check match the other class checks in
  for mergeFrom/isEqual/etc.

InternalDeallocate to avoid confusing the compiler when there's
a class named Element already defined in the global namespace.

Allow unknown values for Map put*Value methods just like every other …

Fix map serialization

Add include for INT_MAX

* Use PyUnicode_AsEncodedString() instead of PyUnicode_AsEncodedObject()

* Cherrypick the fix descriptor_pool.FindFileContainingSymbol by extensions.