WL#5793: Remove THD from Binlog class

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 
Binlog class. This is needed because THD is not stable.

We also add functions to core that read any information that Binlog needs from 
MYSQL_THD objects.

The sub-task of removing THD from Binlog::log_statement is expected to be 
relatively big, and has will therefore be done in a separate worklog: WL#5792.