5346 - add logic to get validators if the block does not exist in the chain
requested to merge 5346-add-logic-to-verify-header into 5346-add-val-to-tendermint-extra-and-verify-header
from the case https://github.com/Evrynetlabs/evrynet-node/pull/18#pullrequestreview-360269596 i ve created this PR to resolves it.