require using grpc_slice

"proto" and "*.egg-info/" are no longer generated, "src/" can be
narrowed to "/src/", and "*_pb2.py" and "*_pb2_grpc.py" are covered by
.gitignore files in parent directories.

- In C++, we need a constant for the max lifetime.
- In C, make sure that we crop the lifetime in the credentials object
itself and not just later during the creation of the token. This will
allow the refresh to occur based on the actual token lifetime as opposed
to the one from the user (which may be cropped).

This reverts commit 53e6b56e.

This reverts commit 3d04e025.