Delete unused code and feature request

Deleted unused code:
I found unused constants and ForkManager. Can I delete them?

Added port number for multi-instance:
When we use MySQL multi-instance, we need port number information to identify instance.

Added --orig_master_is_new_slave for master_ip_online_change_script:
Now I parse command line of parent process to check this option in master_ip_online_change_script.
I would like to get this flag directly.