[refactor] change orphan txs std::map member to use TxId instead of uint256

Summary:
for type safety. as long as i'm touching those lines, I also added
auto liberally and const where appropriate

Test Plan:

ninja check-extended

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6219