[Bitcoin-ABC/bitcoin-abc] Use output-then-input block validation before fork (with tests) (#244)

Preliminary n=1 benchmark results: a 300 second bitcoind —reindex-chainstate run gets to progress=0.018081 with the previous commit, and to progress=0.017903 with commit e292694, a 1% reduction in performance. That slowdown very well could be OS caching effects (I did the e292694 test first) or me doing more webbrowsing during the second test. It also could be a real performance hit due to the position memos. In any case, performance is not substantially worse with this commit, at least for early blocks.