Add accessors for transaction id and n in COutPoint

Summary: This is a first step toward getting more of the codebase to use TxId. Doing the migration is hard when all kind of code touches the fields directly, so the first action item is to go through constructor/accessors.

Test Plan:

make check

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1388