rgw: separate civetweb log from rgw log

The civetweb log now is independent from the rgw log.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: use a separate callback for civetweb access log

Access log is separate from the regular civetweb logging. Also, changed
the log level for the regular logging as it's used mostly for error
messages.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>