(shutdwn): Take exclusive update lock for shutdown. This makes the

(shutdwn): Take exclusive update lock for shutdown.  This makes the
shutdown block until all aborted commands terminated and gave up their
lock.  Remove the cheesy one second wait for commands to finish.  This
also locks out further commands to execute during shutdown.  However,
it also makes it impossible to kill hung player threads here.  Rely on
player_kill_idle() for that.  That doesn't quite work right now,
because it leaves a stale shared lock behind, which blocks shutdown.