Adapt session timeout checks to a per thread model

Each thread will now check their own list of DCBs for timed out sessions.

include/maxscale/dcb.h

@@ -377,6 +377,8 @@ int dcb_accept_SSL(DCB* dcb);
 int dcb_connect_SSL(DCB* dcb);
 int dcb_listen(DCB *listener, const char *config, const char *protocol_name);
 void dcb_append_readqueue(DCB *dcb, GWBUF *buffer);
+void dcb_enable_session_timeouts();
+void dcb_process_idle_sessions(int thr);
 
 /**
  * DCB flags values