Make COutPoint's fields private

Summary:
As per title.

Depends on D1389, D1390, D1391, D1392, D1393, D1394, D1395, D1396, D1397, D1398, D1399, D1401, D1402

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1403