require using grpc_slice

Disbable a specific check on async_end2end_test and revisit later if that expectation was really required by the API

- 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 target is used internally.

This reverts commit 3d04e025.

or that interface with core and are only for internal use