Add Round.undo function

[yatki]

Expected behavior

We'll need to have an undo method in Round module to revert the operations that have been done in apply method.

Actual behavior

We don't have it in the Rounds module at the moment

Acceptance Criteria

• Decrease producedBlocks on mem_accounts table
• Undo votes to mem_accounts table
• If the block is the last of the round, revert rewards & missedBlocks information.
• functions needs to be exported from submodules/round/round.js file for testing but only undo must be exported from submodules/round/index.js.

Which version(s) does this affect? (Environment, OS, etc...)