refactor getrawtransaction fixtures

- transactions hashes are generated with transaction hex => hashes are different for each scenario (https://github.com/JahPowerBit/counterpartyd/blob/develop/test/util_test.py#L88)
- raw transactions are stored in a common sqllite database used as classical pytest read only fixture => no need to have raw transactions in each scenario sql dump (https://github.com/JahPowerBit/counterpartyd/blob/develop/test/conftest.py#L32)
- raw transactions database fixture is generated by using specific parameters when running test `py.test --initrawtransactions --saverawtransactions` => tests stay fast except when generating raw transactions fixtures (https://github.com/JahPowerBit/counterpartyd/blob/develop/test/conftest.py#L27)