MEDIUM: sessions: Keep track of which connections are idle.

Instead of keeping track of the number of connections we're responsible for,
keep track of the number of connections we're responsible for that we are
currently considering idling (ie that we are not using, they may be in use
by other sessions), that way we can actually reuse connections when we have
more connections than the max configured.
diff --git a/src/session.c b/src/session.c
index 48a5d9a..9e95b32 100644
--- a/src/session.c
+++ b/src/session.c
@@ -62,7 +62,7 @@
 		HA_ATOMIC_ADD(&totalconn, 1);
 		HA_ATOMIC_ADD(&jobs, 1);
 		LIST_INIT(&sess->srv_list);
-		sess->resp_conns = 0;
+		sess->idle_conns = 0;
 	}
 	return sess;
 }