[MINOR] add flags to indicate when a stream interface is waiting for space/data

It will soon be required to know when a stream interface is waiting for
buffer data or buffer room. Let's add two flags for that.
diff --git a/src/session.c b/src/session.c
index e163fdf..a775ba5 100644
--- a/src/session.c
+++ b/src/session.c
@@ -945,7 +945,8 @@
 		s->rep->flags &= BF_CLEAR_READ & BF_CLEAR_WRITE & BF_CLEAR_TIMEOUT;
 		s->si[0].prev_state = s->si[0].state;
 		s->si[1].prev_state = s->si[1].state;
-		s->si[0].flags = s->si[1].flags = SI_FL_NONE;
+		s->si[0].flags &= ~(SI_FL_ERR|SI_FL_EXP);
+		s->si[1].flags &= ~(SI_FL_ERR|SI_FL_EXP);
 
 		/* Trick: if a request is being waiting for the server to respond,
 		 * and if we know the server can timeout, we don't want the timeout