ConcurrentMessages issue in setDelay()

[lingjshi]

1. https://github.com/william20111/go-thousandeyes/blob/850c5703d1939f3d037a9757a8dd06581eeb4b5c/client.go#L246

We need to handle the case that the ConcurrentMessages list consists of all the past messages.

for i, t := range rate.ConcurrentMessages {
	if t.Sub(now) >= time.Duration(0) {
		rate.ConcurrentMessages = rate.ConcurrentMessages[i:]
		break
	}
        // Need to clear the list if it consists all the past messages, otherwise the delay will be increased for each incoming request.
	if i == (len(rate.ConcurrentMessages) -1) {
		rate.ConcurrentMessages = rate.ConcurrentMessages[:0]
	}
}