bug: potential for inaccurate data due to missing some blocks when using N2N

[jmhrpr]

There is currently an issue which means some blocks (random) are not processed when using N2N, meaning the accuracy of data cannot be guaranteed. When attempting to roll forward to a new block header while following the tip, the server may sometimes respond to the BlockFetch message with a MsgNoBlocks instead of the requested block data, but Scrolls does not handle this case and behaves as if it processed the block. This means that any actions that would have occurred due to the missed blocks will not be reflected in the data in Scrolls.