[Cashtab] [Simplify wallet structure p1] Show desired end state

Summary:
T2663

Wallet state is disorganized due to iterative changes and preservation of artifacts. This stack will better organize how wallet information is stored and ensure legacy wallets are properly migrated.

In part one, nothing is changed -- we just add the desired end state which future diffs will work toward.

Test Plan: Review the new wallet structure and comment if you note any improvements or better approaches.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D12590