Move utility functions in feature_csv_activation.py out of class.

Summary:
Partial backport of core PR11817 (commit db7ffb9)
https://github.com/bitcoin/bitcoin/pull/11817/commits/db7ffb9d1b9dbdf19b49cc397a7d5e31db244214

Depends on D3559

Test Plan:

./test/functional/test_runner.py feature_csv_activation

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Maniphest Tasks: T669

Differential Revision: https://reviews.bitcoinabc.org/D3560