require using grpc_slice

- 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.

or that interface with core and are only for internal use

Re-enable public constructor for ClientAsyncResponseReader to avoid busting client that bypassed code generator. This code is deprecated-on-arrival as it is a performance pessimization. This code path should not be used.

Fixes compilation if building with -Wextra-semi.