Use TxId internally in COutPoint

Summary:
This is a step toward improving type safety by using TxId/TxHash instead of raw uint256

Depends on D1403

Test Plan:

make check

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Subscribers: schancel, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1404