Use main_config init_delay in request_done

Clip child polling timer to the minimum of initial request processing
delay, instead of the hard-coded 1/3 s in request_done.

This could reduce cleanup latency in configurations with low response
windows and high throughput, preventing possible resource exhaustion.
Let client response windows override init_delay

Allow client's response_window lower than main_config.init_delay to
override it. This allows initial request processing delay to track
client response windows, in addition to home server response windows.
Add max_response_timeouts option

Add "max_response_timeouts" - a home server option specifying number of
times replies are allowed to miss "response_window" before the server
enters the zombie period.

This allows more tolerance before transiting to zombie period for lower
response window configurations.