Skip dropping the log part sequence if there's a lock

So that the job completes. The sequence can be deleted later.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -300,11 +300,22 @@ WHERE job_id = $1")
       (list job-id)))))
 
 (define (drop-log-parts-sequence conn job-id)
-  (exec-query
+  (with-postgresql-transaction
    conn
-   (string-append
+   (lambda (conn)
-    "DROP SEQUENCE IF EXISTS "
+     (exec-query conn
-    (log-part-sequence-name job-id))))
+                 "SET LOCAL lock_timeout = '10s'")
+     (with-exception-handler
+         (lambda (exn)
+           (simple-format (current-error-port)
+                          "error when dropping sequence: ~A"
+                          exn))
+       (lambda ()
+         (exec-query conn
+                     (string-append
+                      "DROP SEQUENCE IF EXISTS "
+                      (log-part-sequence-name job-id))))
+       #:unwind? #t))))
 
 (define (vacuum-log-parts-table conn)
   (exec-query