* Make ProcessNewBlock dbp const and update comment
* Switch reindexing to AcceptBlock in-loop and ActivateBestChain afterwards
* Optimize ActivateBestChain for long chains
* Add -reindex-chainstate that does not rebuild block index
* Report reindexing progress in GUI
Some tests in CheckBlockIndex require chainActive.Tip(), but when reindexing, chainActive has not been set on the first call to CheckBlockIndex.
reindex.py starts a node, mines 3 blocks, stops, and reindexes with CheckBlockIndex enabled.