stream: Add stream_run(), stream_run_wait() functions.

SSL, which will be added in an upcoming commit, requires some background
processing, which is best done in a "run" function in our architecture.
This commit adds stream_run() and stream_run_wait() and calls to them from
the places where they will be required.

lib/stream-tcp.c

@@ -90,6 +90,8 @@ struct stream_class tcp_stream_class = {
     NULL,                       /* connect */
     NULL,                       /* recv */
     NULL,                       /* send */
+    NULL,                       /* run */
+    NULL,                       /* run_wait */
     NULL,                       /* wait */
 };