MINOR: server: implement srv_set_stopped()

MINOR: server: implement srv_set_stopped()

This function was extracted from check_set_server_down(). In only
manipulates the server state and does not consider the health checks
at all, nor does it modify their status. It takes a reason message to
report in logs, however it passes NULL when recursing through the
trackers chain.

For now the function is not used, but the goal is to split health
checks status from server status and to be able to change a server's
state regardless of health checks statuses.