Fix potentially wrong logic if bdk_electrum is used with other chain sources

[evanlinjin]

Description

bdk_electrum may have incorrect reorg-resolution (resulting in inconsistent chain state) if it is paired with other chain sources which do not batch-fetch latest blocks.

Also refactored the chain-updating logic to be more understandable.

Notes to the reviewers

This is still WIP. Needs tests, better documentation and PR description.

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