Merge #12762: Make CKeyStore an interface

Summary:
f381299 Move CKeyStore::cs_KeyStore to CBasicKeyStore (João Barbosa)
25eb9f5 Inline CKeyStore::AddKey(const CKey &) in CBasicKeyStore (João Barbosa)

Pull request description:

Made these simplifications while reviewing #12714. This aims to make `CKeyStore` a *pure* interface:
 - no variable members - the mutex is moved to `CBasicKeyStore` which is where it is used;
 - no method implementations - `AddKey(const CKey &)` is moved to `CBasicKeyStore` which is where it is needed.

Tree-SHA512: 84e44f4390c59600e5cefa599b5464e1771c31dd4abc678ef50db8e06ffac778d692860a352918444f8bcd66430634637b6277a818a658721ffc4f381c1c6a90

Backport of Core PR12762
https://github.com/bitcoin/bitcoin/pull/12762/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3900