clientlog: limit response rate

clientlog: limit response rate

When the measured NTP or command request rate of a client exceeds
a threshold, reply only to a small fraction of the requests to reduce
the network traffic. Clients are allowed to send a burst of requests.
Try to detect broken clients which increase the request rate when not
getting replies and suppress the rate limiting for them.

Add ratelimit and cmdratelimit directives to configure the thresholds,
bursts and leak rates independently for NTP and command response rate
limiting. Both are disabled by default. Commands from localhost are
never limited.