MINOR: server: implement srv_set_running()

MINOR: server: implement srv_set_running()

This function was taken from check_set_server_up(). It does not consider
health checks at all and only sets a server up provided it's not in
maintenance. The resulting state may be either RUNNING or STARTING
depending on the presence of a slowstart or not. The state change is
propagated to tracked servers.

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.