Have insert-blocked-builds cache when the partitions exist

To make it more efficient.

guix-data-service/model/blocked-builds.scm

@@ -133,19 +133,26 @@ WHERE NOT EXISTS (
               (list (number->string build-id)
                     build-server-id)))
 
-(define (insert-blocked-builds conn build-server-id data)
-  (define (create-partition)
-    (exec-query
-     conn
-     (string-append
-      "
-CREATE TABLE IF NOT EXISTS blocked_builds_build_server_"
-      (number->string build-server-id) "
-PARTITION OF blocked_builds FOR VALUES IN ("
-      (number->string build-server-id)
-      ")")))
+(define %created-partitions
+  '())
+
+(define (insert-blocked-builds conn build-server-id data)
+  (unless (null? data)
+    (unless (member build-server-id %created-partitions)
+      (exec-query
+       conn
+       (string-append
+        "
+CREATE TABLE IF NOT EXISTS blocked_builds_build_server_"
+        build-server-id "
+PARTITION OF blocked_builds FOR VALUES IN ("
+        build-server-id
+        ")"))
+
+      (set! %created-partitions
+            (cons build-server-id
+                  %created-partitions)))
 
-  (define (try-insert)
     (exec-query
      conn
      (string-append
@@ -169,9 +176,7 @@ VALUES "
 ON CONFLICT DO NOTHING")
      '()))
 
-  (unless (null? data)
-    (create-partitions)
-    (try-insert)))
+  #t)
 
 (define (handle-populating-blocked-builds-for-scheduled-builds conn build-ids)
   (define (get-build-details build-id)