[MINOR] Add active connection list to server

The motivation for this is to allow iteration of all the connections
of a server without the expense of iterating over the global list
of connections.

The first use of this will be to implement an option to close connections
associated with a server when is is marked as being down or in maintenance
mode.
diff --git a/src/peers.c b/src/peers.c
index cdc8318..f253280 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1185,7 +1185,7 @@
 	stream_sock_prepare_interface(&s->si[1]);
 	s->si[1].release = NULL;
 
-	s->srv_conn = NULL;
+	session_del_srv_conn(s);
 	clear_target(&s->target);
 	s->pend_pos = NULL;