Do not read data from header chain

[majecty]

Header chain is not verified. Anyone could generate valid and high score header chain. We should not read data from the header chain.

If we only use the Tendermint consensus, we can remove the header chain.

[kseo]

@majecty what was the original purpose of the header chain?

[majecty]

Verifying the chain before receiving the whole the chain. Since the best blockchain could be different from the best header chain, we should read the data only from the blockchain.

[junha1]

Verifying header chain itself (aka header sync) can be reused in may places. For example the light client. So it would be better if we make separate module out of this after some fix.