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/session.h b/include/types/session.h
index 5678e5c..7539c0c 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -207,6 +207,7 @@
 	void (*srv_error)(struct session *s,	/* the function to call upon unrecoverable server errors (or NULL) */
 			  struct stream_interface *si);
 	unsigned int uniq_id;			/* unique ID used for the traces */
+	char *unique_id;			/* custom unique ID */
 };
 
 /* parameters to configure tracked counters */