Add a MinTimeToReportBandwidth option; make it 0 for testing networks.

This option changes the time for which a bandwidth measurement period must have been in progress before we include it when reporting our observed bandwidth in our descriptors. Without this option, we only consider a time period towards our maximum if it has been running for a full day. Obviously, that's unacceptable for testing networks, where we'd like to get results as soon as possible.

For non-testing networks, I've put a (somewhat arbitrary) 2-hour minimum on the option, since there are traffic analysis concerns with immediate reporting here.

Closes #40337 (closed).