Refactor governance vote expiration mechanism

libraries/chain/database.cpp

libraries/chain/hive_evaluator.cpp

libraries/chain/include/hive/chain/account_object.hpp

libraries/chain/include/hive/chain/database.hpp

libraries/chain/sps_evaluator.cpp

..._api/include/hive/plugins/condenser_api/condenser_api.hpp

...nclude/hive/plugins/database_api/database_api_objects.hpp

libraries/protocol/hardfork.d/0_25.hf

tests/unit/tests/operation_tests.cpp

tests/unit/tests/proposal_tests.cpp