Fix IsInitialBlockDownload which was broken by headers first.

I don't think we want to eroniously think we're caught up when the best tip is far behind the local time (e.g. because we're broken for some reason).

OTOH, there is a ... somewhat corner case that might be wort worring about:  With this proposed code, if the network goes 24 hours without a block (uh what?) everyone running this will stop mining. :P