Support downgrading after recovered keypool witness keys

Summary:
This is the last part backporting Core PR11403 : https://github.com/bitcoin/bitcoin/pull/11403/commits/7ee54fd7c7fa31e3cec382651dd373f7ef2d7327

depends on D3133

Test Plan:

make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3134