Added open(), close(), cleanup() methods to client_state.

Depending on the DBMS client session allocation strategy the
client id may or may not be available when the client_session
is constructed, therefore there should be a method to assign
an id after construction. Close/cleanup methods were added to
clean up open transactions appropriately.

dbsim/db_client_service.cpp

@@ -34,6 +34,7 @@ int db::client_service::rollback()
     assert(ret == 0);
     return ret;
 }
+
 enum wsrep::provider::status
 db::client_service::replay()
 {