WL#5798: Fix interface of Binlog::log_loaded_block

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].

When LOAD DATA INFILE reads a file, it writes the parts of the file in chunks to 
the binary log. Currently, it stores information about the LOAD DATA statement, 
including the THD object, in a special field in struct IO_CACHE*. Then, the 
IO_CACHE is configured so that it calls log_loaded_block before a new block of 
data is loaded into memory. The IO_CACHE is passed as the only argument to 
log_loaded_block.

We should not maintain this kind of spaghetti code, even less build interfaces 
around it. The most obvious problem is that the generic datatype IO_CACHE now 
contains information specific to LOAD DATA INFILE statements. That's just bad 
design. If we would put IO_CACHE in a stable interface, it would mean that (1) we 
cannot fix the bad design of IO_CACHE; (2) we assume IO_CACHE is stable; (3) we 
assume IO_CACHE is used to load data; but we may want to change to something else 
in the future. So we should change the interface to:

 Binlog::log_loaded_block(THD *thd, int length, unsigned char *data);

This step is not required for the prototype of the feature to link 5.6 rpl-lib to 
5.5 core, but it is required before we announce the interface as stable.