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

src/common/scram-common.c

@@ -22,6 +22,9 @@
 #include "common/base64.h"
 #include "common/hmac.h"
 #include "common/scram-common.h"
+#ifndef FRONTEND
+#include "miscadmin.h"
+#endif
 #include "port/pg_bswap.h"
 
 /*
@@ -73,6 +76,14 @@ scram_SaltedPassword(const char *password,
 	/* Subsequent iterations */
 	for (i = 2; i <= iterations; i++)
 	{
+#ifndef FRONTEND
+		/*
+		 * Make sure that this is interruptible as scram_iterations could be
+		 * set to a large value.
+		 */
+		CHECK_FOR_INTERRUPTS();
+#endif
+
 		if (pg_hmac_init(hmac_ctx, (uint8 *) password, password_len) < 0 ||
 			pg_hmac_update(hmac_ctx, (uint8 *) Ui_prev, key_length) < 0 ||
 			pg_hmac_final(hmac_ctx, Ui, key_length) < 0)