Start handle IDs at 1 instead of 0

This prevents accidentally unsubscribing using an uninitialized handle.  For example:

    ex := exchange.New()
    ex.Subscribe(exchange.Topic("t"), firstListener)
    var handle exchange.Handle // Initialized as 0
    ex.Unsubscribe(handle) // Unsubscribes firstListener

This can also happen when `Subscribe` returns an error and sets a handle to `0`