Add CHECK_FOR_INTERRUPTS() in scram_SaltedPassword() for the backend

scram_SaltedPassword() could take a long time to compute when the number
of iterations used is large enough, and this code uses a tight loop to
compute a salted password.

Note that the same issue exists in libpq when using \password and a
large iteration number, but this cannot be interrupted.  A CFI in the
backend is useful for server-side computations, at least.

Backpatch down to 16, where the user-settable GUC scram_iterations has
been added.

Author: Bowen Shi
Reviewed-by: Aleksander Alekseev, Daniel Gustafsson
Discussion: https://postgr.es/m/CAM_vCueV6xfr08KczfaCEk5J_qeTZtgqN7+orkNLx=g+phE82Q@mail.gmail.com
Backpatch-through: 16

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