Fixes for TensorFlow compilation and running problems

[petewarden]

These are the changes I've found necessary to successfully build and run the top-of-tree protobuf with TensorFlow. I don't understand the intricacies of the Python importing changes, those fixes come from empirically trying different import paths until I found one that worked!

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok to test.

[davidzchen]

I think the changes to add std:: to the using statements are needed, but the imports attribute of Bazel's Python rules should cover adding the right directory to the PYTHONPATH, and the changes to the Python imports here are not necessary.

I have verified that all tests are passing when running baze test //tensorflow/python/... with my patch to replace the vendored protobuf tree in TensorFlow with a workspace dependency in tensorflow/tensorflow#1289.

However, if you happen to have protobuf installed on your local system and are getting Python build errors, most likely, you are running into bazelbuild/bazel#1209.

[davidzchen]

FYI I have opened #1586 for the Python fixes.