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]. We replace function arguments of type THD* by MYSQL_THD* for all functions the Slave class. This is needed because THD is not stable. We also add functions to core that read any information that Slave needs from MYSQL_THD objects.
