`RewindBlockIndex` was made to allow nodes upgrading to new consensus rules post-segwit fork to 
download new consensus data (i.e witnesses) without having to go through a whole IBD. It has been pointed than now really few pre-segwit nodes care to upgrade and that this logic may be irrelevant.

See https://github.com/bitcoin/bitcoin/pull/17737#discussion_r358514335 in review  of #17737 for more context pointers.
INIT_REINDEX,
    INIT_DOWNLOAD,
    POST_INIT
};
extern RecursiveMutex cs_main;
typedef std::unordered_map<uint256, CBlockIndex*, BlockHasher> BlockMap;
extern Mutex g_best_block_mutex;
extern std::condition_variable g_best_block_cv;
extern uint256 g_best_block;
extern std::atomic_bool fImporting;
extern std::atomic_bool fReindex;
/** Whether there are dedicated script-checking threads running.
 * False indicates all script checking is done on the main threadMessageHandler thread.
 */
extern bool g_parallel_script_checks;
extern bool fRequireStandard;
extern bool fCheckBlockIndex;
extern bool fCheckpointsEnabled;