Fix block construction in abc-p2p-compactblocks.py
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.
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D2104