Make bip68-112-113-p2p.py conform to magnetic anomaly

Summary:
Remove magnetic anomaly activation in the future, make blocks conform to
CTOR and pad transactions.

Depends on D2176

Test Plan:

./test/functional/bip68-112-113-p2p.py

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2177