Improve low-latency link handling

These are the rest of the patches improving handling of low-latency links, finishing the work started in https://github.com/FreeRADIUS/freeradius-server/pull/645.
They were tested by @jons-rt-dev at Syniverse.

I'm not sure about the necessity of "Use main_config init_delay in request_done", but it seems the right thing to do. The other two changes are required for Syniverse deployment.