Add FreezeLog functionality

[YoshihitoAso]

• Close: #582
• Add following APIs.
  • POST: /freeze_log/accounts
  • Creation of an account (private key) for logging.
  • The private key is stored in the same format as the issuer key.
  • GET: /freeze_log/accounts
  • List all accounts.
  • DELETE: /freeze_log/accounts/{account_address}
  • Delete a logging account.
  • POST: /freeze_log/accounts/{account_address}/eoa_password
  • Change EOA kefile password.
  • POST: /freeze_log/logs
  • Record new log message.
  • POST: /freeze_log/logs/{log_index}
  • Update log message.
  • GET: /freeze_log/logs/{log_index}
  • Retrieve stored log.