WL#1227: SHOW PROCESSLIST: Add better output for slave processing LOAD DATA

Affects: Server-7.1   —   Status: Assigned

Current output of SHOW PROCESSLIST for slave processing initial
events for LOAD DATA (Create_file, Append_block) is not very 
descriptive. So sometimes user should guess "What slave thread 
is doing now ?"

The idea is to display "Appending block to temporary file 'xxx'" info for 
Create_file and Append_block events. We even can put size of this file in
 this string to show progress.

For Exec_load_log_event we could display the complete query.

Citing Guilhem: To do this, we just need to call some code of
Load_log_event::print(). Redirect the output in a string, and you'll get a
beautiful SHOW PROCESSLIST when the slave is executing Exec_load.