Use block requests to check if block responses are correct

Before this pr sync relied on recently announced blocks to check if a
given peer response is correct. However this could lead to situations
where we requested a block from a peer and it gave us the requested, but
we rejected the response because this peer never send us an announcement
for the given block. See the added tests for a reproduction of the
problem.
With this pr, we now take the block request to check if a given response
matches the request. A node should not send us a block response
without a request anyway.

Essentially there is still a bug, because as you see in the test, we are
requesting block 2, while we already have this block imported. It even
happens that we request a block from the network that we have authored.
However a fix for this would require some more refactoring of the sync code.