MINOR: stream_interface: Give stream_interface its own wait_list.

Instead of just using the conn_stream wait_list, give the stream_interface
its own. When the conn_stream will have its own buffers, the stream_interface
may have to wait on it.
diff --git a/include/types/stream_interface.h b/include/types/stream_interface.h
index 0c83759..eae1926 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -101,6 +101,7 @@
 	unsigned int err_type;  /* first error detected, one of SI_ET_* */
 	int conn_retries;	/* number of connect retries left */
 	unsigned int hcto;      /* half-closed timeout (0 = unset) */
+	struct wait_list wait_list; /* We're in a wait list */
 };
 
 /* operations available on a stream-interface */