CKeystore/CCrypter: move relevant implementation out of the header

Summary:
This is a refactoring backport, no functional change is made here.

Backport of core PR11272

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2408