Tweak fetching narinfos

Currently I'm seeing failures due to guile-gnutls not supporting suspendable
ports (write_wait_fd), so batch the requested outputs to try and avoid this.

guix-data-service/substitutes.scm

@@ -22,6 +22,7 @@
   #:use-module (ice-9 threads)
   #:use-module (guix substitutes)
   #:use-module (guix narinfo)
+  #:use-module (guix-data-service utils)
   #:use-module (guix-data-service database)
   #:use-module (guix-data-service model build-server)
   #:use-module (guix-data-service model git-branch)
@@ -85,8 +86,11 @@
   (simple-format #t "Querying ~A outputs\n"
                  (length outputs))
 
+  (chunk-for-each!
+   (lambda (outputs-chunk)
      (let ((narinfos
-         (lookup-narinfos (string-trim-right build-server-url #\/) outputs)))
+            (lookup-narinfos (string-trim-right build-server-url #\/)
+                             outputs-chunk)))
 
        (simple-format #t "Got ~A narinfo files\n"
                       (length narinfos))
@@ -113,6 +117,8 @@
                conn
                build-server-id
                filtered-narinfos)))))))
+   2000
+   outputs))
 
 (define (start-substitute-query-thread)
   (call-with-new-thread