MINOR: stream: Add stream_generate_unique_id function

Currently unique IDs for a stream are generated using repetitive code in
multiple locations, possibly allowing for inconsistent behavior.
diff --git a/include/proto/stream.h b/include/proto/stream.h
index f8c0887..e54ac60 100644
--- a/include/proto/stream.h
+++ b/include/proto/stream.h
@@ -53,6 +53,7 @@
 #define IS_HTX_STRM(strm) ((strm)->flags & SF_HTX)
 
 extern struct pool_head *pool_head_stream;
+extern struct pool_head *pool_head_uniqueid;
 extern struct list streams;
 
 extern struct data_cb sess_conn_cb;
@@ -65,6 +66,8 @@
 void stream_dump(struct buffer *buf, const struct stream *s, const char *pfx, char eol);
 void stream_dump_and_crash(enum obj_type *obj, int rate);
 
+int stream_generate_unique_id(struct stream *strm, struct list *format);
+
 void stream_process_counters(struct stream *s);
 void sess_change_server(struct stream *sess, struct server *newsrv);
 struct task *process_stream(struct task *t, void *context, unsigned short state);