This is a step towards making replication a library, such that the 5.6 rpl-lib can be plugged into the 5.5 core [WL#5675]. In particular, this is a step toward removing function arguments of type THD* from the interface. This is needed because THD is not stable. In this worklog, we make the class function Binlog::log_statement() take a list of user variables as one parameter. Before this worklog, user variables were read from the THD argument. See also WL#5790.