Ensure that non-standalone supervisor terminates gracefully

Standalone supervisors run this from the signal handler, so for
non-standalone we need to make sure this is run after shutting down.

Author: rosa

lib/solid_queue/async_supervisor.rb

@@ -2,6 +2,8 @@
 
 module SolidQueue
   class AsyncSupervisor < Supervisor
+    after_shutdown :terminate_gracefully, unless: :standalone?
+
     def stop
       super
       @thread&.join