Add on_demand Watcher option

The patch allows circus to simulate "inetd-style" on demand process spawning.

If a watcher is configured as "on_demand", its processes will be only started at the first request made to the socket.

The sockets are monitored via standard select() (i do not think it is worthy to add kqueue/epoll support as i suppose it will be uncommon to monitor hundreds of file descriptors in the same arbiter)