Make -reindex cope with out-of-order blocks
Make -reindex cope with out-of-order blocks

Remember out-of-order block headers along with disk positions. This is
likely the simplest and least-impact way to make -reindex work with
headers first.

Based on top of #4468.

wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin