Canonicalization should handle transactions that spend from two conflicting transactions

[evanlinjin]

Fixes #1898

Description

When we initially created the canonicalization algorithm, we didn't expect callers to insert invalid transactions into the graph. However, user error happens and we should handle it correctly.

Right now, when you insert a transaction that spends from two conflicting transactions, the canonicalization result will have duplicates. Let's fix this here (starting with a set of tests).

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