[mod] limiter: ip_limt - monitore suspicious IPs

To intercept bots that get their IPs from a range of IPs, there is a
``SUSPICIOUS_IP_WINDOW``.  In this window the suspicious IPs are stored for a
longer time.  IPs stored in this sliding window have a maximum of
``SUSPICIOUS_IP_MAX`` accesses before they are blocked.  As soon as the IP makes
a request that is not suspicious, the sliding window for this IP is droped.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>