Stop keeping track of epoch changes for the sync gap

[davxy]

Epoch changes data for the gap (i.e. the blocks below the warp sync point) was used to verify the blocks imported after a warp sync.

Currently this verification is not really required since all our security depends on the last finalized block.

Once the last finalized block is imported (i.e. the warp sync point) then we start importing all the previous blocks and we can detect a bad history only after we finished importing the block before the warp sync point.

We may think that in this way we may be tricked by a malicious full node into downloading an history of blocks that in the end we find out to not be good WRT the finalized block we warp synced to. However, the same type of attack can be performed by one of the initial authorities by feeding an history that is coherently signed together with a crafted dummy authority set changes.

Thus our only stable grip is the last finalized block anyway. This is the point that in the end will allow us to distinguish between a good history from a bad one.

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Note. In the future we may think to early detect this type of annoyance by downloading the blocks in reverse order. In this way, for each block we can immediately check if the header hash is equal to the parent of the block that follows.

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Fixes zombienet test 0003 in #13078

[melekes]

👍 (code-wise; don't know enough to reason about business logic)