software/rapid-cdn: Implement frontend log

The access log is useful for usual analysis, but CDN has to provide deep
metrics regarding each request, thus implement the frontend log, which follows
more or less HAProxy's httplog
http://docs.haproxy.org/2.6/configuration.html#option%20httplog and can be
correlated with a timestamp to access log.

The URI string is dropped from the frontend log, as it's already supported by
the access log. Thanks to this it's safe to use !!! (triple exclamation mark)
to split the log line, as those characters are not expected to be found in
usual metrics provided by HAProxy.