C API to change effective user
We would like to have a C API for switching the effective user without changing the session user in the fiber. At the moment the session credentials are allocated inside the session memory pool, while the effective user refers them (or can be changed for a short time with some other credentials on the stack as we do in lbox_session_su). Looks like we need either to extend the fiber's struct size and inline the effective user struct inside, or create some new credentials memory pool...