loginmanager: Associate ScheduledExecutor with the cell's thread group

Would lead to log messages to be associated with the wrong cell. Also
caused issues during shutdown.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7382/