Investigate: Genesis syncing with exactly one peer is slow

@karknu reports that syncing with the following topology file with Genesis enabled is very slow:

> Slow as in kbyte per second, It seems to stall for minutes at the time

We should look at this at some point. It is relatively low-prio as it doesn't make a lot of sense to sync with exactly one peer with Genesis (as one must trust this singular peer, so one could also just use Praos mode), but it is still surprising (especially if/when Genesis mode becomes the default in the future) and can maybe improve things even in the multi-peer case.

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Historically, while developing Genesis, we observed such behavior (syncing with exactly one peer being much slower than syncing with more than one peer) due to a peculiar interaction between GDD and CSJ, but this *should* have been resolved via the GDD rate limit.