- 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 3d04e025.

or that interface with core and are only for internal use

- Make sure all C-core parameters are passed into C++ methods.
- Add Destroy() methods for ChannelData and CallData.
- Use C++-style casts.
- Add 'extern "C"' to iomgr/closure.h, which is used in C++ filters.

MS Visual studio '13 and before don't understand inline
and throw Error C2054.
Reference: https://msdn.microsoft.com/en-us/library/bw1hbe6y.aspx