Timeout when trying to process build events

As if they relate to derivations that are currently being inserted, they'll
have to wait until the job is processed.

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

@@ -20,6 +20,7 @@
   #:use-module (ice-9 match)
   #:use-module (rnrs bytevectors)
   #:use-module (json)
+  #:use-module (squee)
   #:use-module (fibers)
   #:use-module (prometheus)
   #:use-module (guix-data-service utils)
@@ -218,6 +219,8 @@
              (with-postgresql-transaction
               conn
               (lambda (conn)
+                (exec-query conn "SET LOCAL lock_timeout = '5s';")
+
                 (handle-derivation-events
                  conn
                  filtered-items))))))