Add OpenTerm transaction

[sgkim126]

Currently, when a 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.

OpenTerm transaction is one of these. This transaction should include:

• the next term number
• newly elected validators
• rewards each validator will receive
• the new list of new candidates which the expired accounts are removed from
• newly jailed accounts
• the list of released prisoners
• the delegations that are withdrew