Force Persisted<T> to mutate and persist atomically

[evanlinjin]

Description

@tnull asked why we have to do a separate .persist call when using Persisted<Wallet>. This is my solution to not having to do that.

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[tnull]

Thank you for looking into this! It seems like a viable option even though it somewhat complicates the wallet API.

Although, as noted on Discord, I personally might even prefer the current simpler API as long as it clearly communicates when persistence should/may happen (e.g., would return a result along the lines of enum PendingPersistence { Immediately, Eventually } for all codepaths that need persistence), and the docs would provide clear guidance how to handle persistence failures.