Increase block download timeout base from 10 to 20 minutes.

This harmonizes the block fetch timeout with the existing ping timeout
and eliminates a guaranteed eventual failure from congestion collapse
for a network operating right at its limit.

It's unlikely that we wouldn't suffer other failures if we were really
anywhere near the network's limit, and a complete avoidance of congestion
collapse risk requires (I think) an exponential back-off. So this isn't
a major concern, but I think it's also useful for reducing the complexity
of understanding out timeouts.