fix(wallet): Don't fail in build_fee_bump for missing parent txid

[ValuedMammal]

Description

This is a draft PR refactoring build_fee_bump in attempt to resolve #325.

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

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

[coveralls]

Pull Request Test Coverage Report for Build 18917041894

Details

• 43 of 43 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 85.136%

---

Totals
Change from base Build 18891447990:	0.1%
Covered Lines:	7005
Relevant Lines:	8228

---

💛 - Coveralls