Clamp init_delay to half the minimum response window 

Update main_config init_delay with half the minimum response window
across all home servers, if it's less, instead of the whole.

This makes proxy timeouts match the response windows near to and lower
than 1/3 of a second, which fixes issue #673.

We've verified these response windows to work correctly:

1.2
0.8
0.6
0.4
0.2
0.04
0.02