MEDIUM: http: add support for action "set-log-level" in http-request/http-response

MEDIUM: http: add support for action "set-log-level" in http-request/http-response

Some users want to disable logging for certain non-important requests such as
stats requests or health-checks coming from another equipment. Other users want
to log with a higher importance (eg: notice) some special traffic (POST requests,
authenticated requests, requests coming from suspicious IPs) or some abnormally
large responses.

This patch responds to all these needs at once by adding a "set-log-level" action
to http-request/http-response. The 8 syslog levels are supported, as well as "silent"
to disable logging.