Split SetHDChain into AddHDChain and LoadHDChain

Summary:
Remove the memonly bool and follow our typical Add and Load pattern.

This is a backport of core#19046 [1/3]
https://github.com/bitcoin/bitcoin/pull/19046/commits/0122fbab4c340b23ae56173de6c5ab866ba25ab8

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9630