Add CloseBlock transaction

[sgkim126]

Currently, when the term is open, each validator got paid the pending rewards, and the network elects new validators. Because the nodes calculate the rewards implicitly, applications (e.g., the indexer) must calculate the rewards by themselves or query the states every block.

To remove this inconvenience, #40 proposes to remove implicit state changes.

The CloseBlock transaction is one of these. This transaction should include,

• the block author who will receive the rewards
• the sum of fee in the block