Use the connection pool for build events

I'm not sure why this wasn't the way it worked previously. This should avoid
unconstrained use of PostgreSQL connections.

guix-data-service/web/build-server/controller.scm

@@ -121,23 +121,21 @@
   (define (spawn-fiber-for-handler handler)
     (spawn-fiber
      (lambda ()
-       (with-postgresql-connection
-        "build-event-handler-conn"
-        (lambda (conn)
-          (with-exception-handler
-              (lambda (exn)
-                (simple-format
-                 (current-error-port)
-                 "exception in build event handler: ~A\n"
-                 exn))
-            (lambda ()
-              (with-throw-handler #t
-                (lambda ()
-                  (handler conn))
-                (lambda _
-                  (display (backtrace) (current-error-port))
-                  (display "\n" (current-error-port)))))
-            #:unwind? #t))))))
+       (with-resource-from-pool (connection-pool) conn
+         (with-exception-handler
+             (lambda (exn)
+               (simple-format
+                (current-error-port)
+                "exception in build event handler: ~A\n"
+                exn))
+           (lambda ()
+             (with-throw-handler #t
+               (lambda ()
+                 (handler conn))
+               (lambda _
+                 (display (backtrace) (current-error-port))
+                 (display "\n" (current-error-port)))))
+           #:unwind? #t)))))
 
   (define (with-build-ids-for-status data
                                      build-ids