IS NOT USED. 

Object "err_log_throttle" was created to avoid spamming error
log file on failure to create thread for new connection. After
moving logic to add new connection to "Per_thread_connection_handler
::add_connection", this object was not used. Removed this object
now. Also changed code to log error message on failure to create 
thread in "Per_thread_connection_handler::add_connection" function
and used new static member "ERROR_LOG_THROTTLE 
create_thd_err_log_throttle" in file "connection_handler_per_thread.cc"
to avoid spamming error log file on thread creation failure for new
connection. This restores the functionality.