[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/include/types/session.h b/include/types/session.h
index 2ac2232..7fde0aa 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -157,6 +157,7 @@
  */
 struct session {
 	struct list list;			/* position in global sessions list */
+	struct list by_srv;			/* position in server session list */
 	struct list back_refs;			/* list of users tracking this session */
 	struct task *task;			/* the task associated with this session */
 	/* application specific below */