Fix block construction in abc-p2p-compactblocks.py

Summary:
Because of padding, the code generation a block may have trouble ensuring the block size is the right one. Update such code to take advantage of functionallities added in pad_tx in D2098 to ensure the size is correct.

Depends on D2098, D2103

Test Plan:

./test/functional/test_runner.py abc-p2p-compactblock.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2104