Malicious slowdown using valid headers

[MaksymZavershynskyi]

Suppose there is NearOnEthClient running on Ethereum blockchain that accepts headers every 4 hours from Near2EthRelay. Suppose the last header in NearOnEthClient hash height X, as soon as it passes the challenge period someone malicious submits a valid header of height X+1. This header is valid and cannot be challenged, but it progresses client only 1 block ahead. In 4 hours the attack can be repeated.

The solution proposed by @abacabadabacaba is the following:

Allow replacing the current block during challenge period if the new block is at least 5 hours ahead of the current block. When this happens, the challenge period is restarted.