ovsdb: Make JSON-RPC sessions other than the first work.

The number of open sessions wasn't being tracked properly, so closing a
JSON-RPC database session made it impossible to open any more.

ovsdb/jsonrpc-server.c

@@ -263,6 +263,7 @@ ovsdb_jsonrpc_session_open(struct ovsdb_jsonrpc_server *svr,
     s->rpc = jsonrpc_open(stream);
     hmap_init(&s->triggers);
     list_init(&s->completions);
+    svr->n_sessions++;
 }
 
 static void