MINOR: fd: make fdtab->owner a connection and not a stream_interface anymore

It is more convenient with a connection here and will abstract stream_interface
more easily.
diff --git a/src/session.c b/src/session.c
index 0e6c11e..998270b 100644
--- a/src/session.c
+++ b/src/session.c
@@ -281,7 +281,7 @@
 
 	/* finish initialization of the accepted file descriptor */
 	fd_insert(cfd);
-	fdtab[cfd].owner = &s->si[0];
+	fdtab[cfd].owner = &s->si[0].conn;
 	fdtab[cfd].flags = 0;
 	fdtab[cfd].cb[DIR_RD].f = NULL;
 	fdtab[cfd].cb[DIR_WR].f = NULL;