Allow for the downloading of initial headers from one pruned node.
Allow for the downloading of initial headers from one pruned node.

This fixes a nuisance bug where we may have just one connection to
a pruned now while testing. This can happen when we use the connect=<ip>
and do not allow any inbound peers, in which case we would end up
not being able to sync the chain if we were behind by less than
a day's (144) blocks.

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