net: Use mockable time for tx download
Summary:
Two commits: - First commit changes to mockable time for tx download (refactoring, should only have an effect on regtest) - Second commit adds a test that uses mocktime to test tx download
Backport of core PR16197.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D6154