Implement CWallet::IsSpentKey for non-LegacySPKMans
Summary: Backport of Core PR16528 [40/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/886e0d75f5fea2421190aa4812777d89f68962cc
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D8466