[backport#15777] [wallet] move-only: move CReserveKey to be next to CKeyPool

Summary:
reviewer tip: use git diff --color-moved=dimmed-zebra

https://github.com/bitcoin/bitcoin/pull/15777/commits/ef2d515af3c74f61acca5c0c8936a30e7befc507

Partial backport of Core PR15777

Test Plan:

ninja

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6348