Refactor duplicate input check

Summary:

• Use std::unordered_set instead of std::set.
• Do not try to check duplicate input for coinbase.
• Always check duplicate input for regular transactions.

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1790