[qa] fix transaction ordering and padding in abc-p2p-compactblocks.py

Summary:
Fix abc-p2p-compactblocks.py to support magnetic anomaly upgrade's transaction ordering and padding requirements.

Depends on D2086

Test Plan: test_runner.py abc-p2p-compactblocks

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2079