Implement verifyBlockForger in Dpos module

[lsilvs]

Description

Dpos module should expose a method to validate if a given block was forged by the expected delegate.

Motivation

As Dpos module is replacing Rounds module, this method is required to replace round. validateBlockSlot and round.validateBlockSlotAgainstPreviousRound

Acceptance Criteria

The new method should validate if a block was forged by the expected delegate for any given block that was not finalized yet.

Additional Information

feature/bft

[michielmulders]

Before finalizedheight, we don't have the delegate list anymore. We only store up to finalizedheight. (Check first if block height is between last finalized and current block - first simple check -> If not, throw this block is older than the last finalized height and cannot be validated and then the full check.)

Note: Combine both methods into one!

[shuse2]

@michielmulders I think dpos doesn't know anything about finalized (it is property of bft), im not sure how to check or when we need to check.
I think it make more sense to check if the checking round exists, and if not just throw an error?