• Adds build_shared() to the abstract builder, so that the caller doesn't have to specify std::shared_ptr<nano::state_block> ..., using auto instead. Keeps tests formatting clean.
• Use block builders in all active_transactions and node tests from core_test

Used the following script used to convert (95% of cases work without modifications): https://gist.github.com/guilhermelawless/6424ea9199db7584e314381cbce5dc65