[backport#16227 2/8] Remove CKeyStore and squash into CBasicKeyStore

Summary:
https://github.com/bitcoin/bitcoin/pull/16227/commits/c7797ec65544bd23a2e571b2892e1bf512f2a485

Depends on D6553

Partial backport of Core PR16227

Test Plan:

ninja check

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6554