WL#5792: Remove THD from prototype for Binlog::log_statement

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 remove the function argument of type THD* from the Binlog::log_statement. This 
is needed because THD is not stable. Instead, we do:
 - In rpl-lib, add argument of type MYSQL_THD to Binlog::log_statement
 - In core, add thd_ functions that get the information that Binlog::log_statement
   needs.
 - In rpl-lib, add more arguments to Binlog::log_statement if needed.