[MEDIUM] session: move the conn_retries attribute to the stream interface

The conn_retries still lies in the session and its initialization depends
on the backend when it may not yet be known. Let's first move it to the
stream interface.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index bbda255..dee7aa4 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -2540,7 +2540,7 @@
 
 		chunk_printf(&msg,
 			     "  flags=0x%x, conn_retries=%d, srv_conn=%p, pend_pos=%p\n",
-			     sess->flags, sess->conn_retries, sess->srv_conn, sess->pend_pos);
+			     sess->flags, sess->si[1].conn_retries, sess->srv_conn, sess->pend_pos);
 
 		chunk_printf(&msg,
 			     "  frontend=%s (id=%u mode=%s), listener=%s (id=%u)\n",