MEDIUM: stream: Make the `unique_id` member of `struct stream` a `struct ist`

The `unique_id` member of `struct stream` now is a `struct ist`.
diff --git a/include/types/stream.h b/include/types/stream.h
index 90819e6..39a6855 100644
--- a/include/types/stream.h
+++ b/include/types/stream.h
@@ -177,7 +177,7 @@
 	int pcli_next_pid;                      /* next target PID to use for the CLI proxy */
 	int pcli_flags;                         /* flags for CLI proxy */
 
-	char *unique_id;                        /* custom unique ID */
+	struct ist unique_id;                   /* custom unique ID */
 
 	/* These two pointers are used to resume the execution of the rule lists. */
 	struct list *current_rule_list;         /* this is used to store the current executed rule list. */