Bug #17357535 BACKPORT BUG#16241992 TO 5.5

Problem:
COM_CHANGE_USER allows brute-force attempts to crack a password at a very high
rate as it does not cause any significant delay after a login attempt has
failed. This issue was reproduced using John-The-Ripper password
cracking tool through which about 5000 passwords per second could be attempted.

Solution:
The non-GA version's solution was to disconnect the connection when a login
attempt failed. Now since our aim to to reduce the rate at which passwords 
are tested, we introduced a sleep(1) after every login attempt failed. This
significantly increased the delay with which the password was cracked.

sql/sql_parse.cc

@@ -971,6 +971,7 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
       thd->variables.collation_connection= save_collation_connection;
       thd->variables.character_set_results= save_character_set_results;
       thd->update_charset();
+      sleep(1);
     }
     else
     {