Remove adaptive, keep stats for deny rules, add clearing stats

Remove adaptive, keep stats for deny rules, add clearing stats

Remove so called adaptive rules, which meant moving currently matched
deny rules to the front of @filter_deny list.  Better solution I think
would be to rely on rule usage statistics.

Extend deny rule usage statistics, adding average number of rules to
match (excluding cases where there was no match at all).

Add '/FILTER CLEARSTATS' command, to clear (reset) statistics for
rules.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>