Best proposal header could not exist

[majecty]

Best proposal header has a significant role in our Tendermint's liveness.

When less than 2/3 of committee members are in the height h and the first proposer is in the height h - 1, the proposer cannot move to the height h beacuse there is no best proposal.

[majecty]

By adding commit message #1770, we don't depend on the best proposal header for liveness.