Fixed p2p-compactblocks failures

Summary: CTOR changes tx ordering, so selecting hand-picked tx indices in a block is no longer possible. Fixes this + tx padding

Test Plan: test_runner.py p2p-compactblocks

Reviewers: deadalnix, schancel, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2078