Ensure the thresold for BIP9 can be configured on a per activation basis.

Summary:
This gives more flexibility.

Depends on D5135

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5139