Memoize computing tokens

As I'm not sure how expensive this is, but it doesn't need doing for every
request.

guix-data-service/model/build-server-token-seed.scm

@@ -21,21 +21,24 @@
   #:use-module (squee)
   #:use-module (gcrypt hash)
   #:use-module (gcrypt base64)
+  #:use-module (guix memoization)
   #:export (compute-tokens-for-build-server))
 
-(define (compute-token secret-key-base build-server-id token-seed)
-  (let ((source-string
-         (simple-format #f "~A:~A:~A"
-                        secret-key-base
-                        build-server-id
-                        token-seed)))
-    (string-filter
-     ;; Remove the + / and = to make handling the value easier
-     char-set:letter+digit
-     (base64-encode
-      (bytevector-hash
-       (string->utf8 source-string)
-       (hash-algorithm sha1))))))
+(define compute-token
+  (memoize
+   (lambda (secret-key-base build-server-id token-seed)
+     (let ((source-string
+            (simple-format #f "~A:~A:~A"
+                           secret-key-base
+                           build-server-id
+                           token-seed)))
+       (string-filter
+        ;; Remove the + / and = to make handling the value easier
+        char-set:letter+digit
+        (base64-encode
+         (bytevector-hash
+          (string->utf8 source-string)
+          (hash-algorithm sha1))))))))
 
 (define (compute-tokens-for-build-server conn secret-key-base build-server-id)
   (define query