Some simplification in the compact block code and tests
Summary: As per title.
Test Plan:
make check ../qa/pull-tester/rpc-tests.py
Reviewers: freetrader, sickpig, kyuupichan, awemany, #bitcoin_abc
Reviewed By: freetrader, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D232