Fix re-org treatment

[junderw]

1. Add new index with block height + block hash + TxHistoryRow entries for the last 100 blocks.
2. When a reorg occurs, utilize that to clean up the TxHistoryRows and roll them back.
3. StatsCache and UtxoCache to only cache up to 6 blocks deep. This will help prevent incorrect stats information.
   a. Not sure what the performance consequences are for this... maybe 3 blocks deep is more acceptable.