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.

Add some new ignores for things generated in conformance.

* Issue #2211: Addressing GCC warning on enumeral/non-enumeral in conditional expression.

* Updated per 80 character wrap.

Added a workaround to allow building for NaCl

arm-nacl-clang seems to produce a mysterious compiler error when it
encounters __builtin_clzll, so when we are building for NaCl, let's
avoid that in favor of the portable implementation of
Log2FloorNonZero64.

Raise the recursion limit to 100 to match other languages.

Changed go_package for plugin.proto.