Make CWallet IsSpent and IsLockedCoin take a COutPoint as parameter

Summary:
These methods currently take a transaction id and the output order,
which is the definition of a COutPoint.

This is a follow up from
https://reviews.bitcoinabc.org/D3420?id=9680#inline-21184

Test Plan:

make check
./test/functional/test_runner.py wallet_*

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3515