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.

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

Fix map serialization

Add include for INT_MAX

* Use PyUnicode_AsEncodedString() instead of PyUnicode_AsEncodedObject()

* Cherrypick the fix descriptor_pool.FindFileContainingSymbol by extensions.

Cleanup reflection objects for map entry.

Update version number and changelog for 3.3.0

See https://github.com/google/protobuf/issues/2425

Remove the use of dispatch_once that is heap backed.

Apple recently updated the docs on dispatch_once to point out
that the storage for the dispatch_once_t must be static or global,
but not something that was ever used before as the implementation
doesn't use a memory barrier.  So we drop the use and create the
semaphore when needed and use an atomic swap deal with any
threading races.

Add support for Xcode 8.3 to the build helper.