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.
Merge pull request #672 from spbnick/init_delay_rename_v3.0.x

Rename min_response_window to init_delay - v3.0.x