[MEDIUM] session-counters: add HTTP req/err tracking

[MEDIUM] session-counters: add HTTP req/err tracking

This patch adds support for the following session counters :
  - http_req_cnt : HTTP request count
  - http_req_rate: HTTP request rate
  - http_err_cnt : HTTP request error count
  - http_err_rate: HTTP request error rate

The equivalent ACLs have been added to check the tracked counters
for the current session or the counters of the current source.