MINOR: server: implement srv_set_stopping()

MINOR: server: implement srv_set_stopping()

This function was taken from check_set_server_drain(). It does not
consider health checks at all and only sets a server to stopping
provided it's not in maintenance and is not currently stopped. The
resulting state will be STOPPING. 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.