- Calls to cleanup_load_tmpdir() could delete temporary files for another master connection
- Concurrent LOAD DATA commands from two master connections could use the same file name

Other bug fixes:
- Enlarge buffer for connection names with 'special characters' one can't store in filenames

Optimization:
- Don't do 'lower case' of connection names. We can use cmp_connection_name, where we already have the connection name in lower case.


mysql-test/suite/multi_source/load_data.result:
  Test case for MDEV-4352
mysql-test/suite/multi_source/load_data.test:
  Test case for MDEV-4352
sql/log_event.cc:
  Fixed: MDEV-4352
  - Calls to cleanup_load_tmpdir() could delete temporary files for another master connection
  - Concurrent LOAD DATA commands from two master connections could use the same file name
  
  The fix was to add the connection name (if one exists) to all slave temporary files used by LOAD DATA
sql/rpl_mi.cc:
  Enlarge buffer for connection names with 'special characters' one can't store in filenames
  Use mi->cmp_connection_name for connection file names.
sql/rpl_rli.cc:
  Use mi->cmp_connection_name for connection file names.
sql/slave.cc:
  Removed not needed empty line
sql/sql_const.h:
  Added MAX_FILENAME_MBWIDTH to be able to calculate buffer length for connection_names stored in file names
sql/sql_repl.cc:
  Use mi->cmp_connection_name for connection file names.