MEDIUM: log: Unique ID

The Unique ID, is an ID generated with several informations. You can use
a log-format string to customize it, with the "unique-id-format" keyword,
and insert it in the request header, with the "unique-id-header" keyword.
diff --git a/include/types/proxy.h b/include/types/proxy.h
index d69a914..aa6cec8 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -290,6 +290,8 @@
 	struct proxy *next;
 	struct list logsrvs;
 	struct list logformat; 			/* log_format linked list */
+	char *header_unique_id; 		/* unique-id header */
+	struct list format_unique_id;		/* unique-id format */
 	int to_log;				/* things to be logged (LW_*) */
 	int stop_time;                          /* date to stop listening, when stopping != 0 (int ticks) */
 	struct hdr_exp *req_exp;		/* regular expressions for request headers */