Reset TOI meta after releasing total order in provider

This is to keep the TOI meta available in case the provider
implementation needs it.

test/mock_provider.hpp

@@ -260,6 +260,7 @@ namespace wsrep
         }
 
         enum wsrep::provider::status leave_toi(wsrep::client_id,
+                                               const wsrep::ws_meta&,
                                                const wsrep::mutable_buffer&)
             WSREP_OVERRIDE
         { return wsrep::provider::success; }