Better conflict/acceptance resolver

[bchamagne]

Description

• download_transaction check for the tx's integrity directly in the acceptance_resolver
• fetch_transaction's conflict_resolver was naive and accepted the first non-error result

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Unit tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules