Rename min_response_window to init_delay

Rename main_config's "min_response_window" field to "init_delay", as it
better reflects the usage and because its value won't be related to any
response window in case they're all higher than 1/3 s, which is true
most of the time.

I've changed my mind back to "init_delay" while working on the changes
you suggested as it seemed to fit better. Applying this wouldn't change
any functionality, but might make code slightly easier to understand later.

Please feel free to reject, if you think otherwise.