blockchain: don't rely on BlockHeightByHash for prune height

calculations

Since BlockHeightByHash only returns the heights for blocks that are in
the main chain, when a block that is stale gets pruned, this will cause
an error in the block height lookup and cause an error in block
processing.

Look up the node directly from the index and if the node isn't found,
just skip that node. For utxoCache.lastFlushHash, if that isn't found,
just force a flush.

blockchain/utxocache.go

@@ -726,10 +726,18 @@ func (b *BlockChain) flushNeededAfterPrune(earliestHeight int32) (bool, error) {
 	if earliestHeight < 0 {
 		return false, nil
 	}
-	lastFlushHeight, err := b.BlockHeightByHash(&b.utxoCache.lastFlushHash)
-	if err != nil {
-		return false, err
-	}
-
+	node := b.index.LookupNode(&b.utxoCache.lastFlushHash)
+	if node == nil {
+		// If we couldn't find the node where we last flushed at, have the utxo cache
+		// flush to be safe and that will set the last flush hash again.
+		//
+		// This realistically should never happen as nodes are never deleted from
+		// the block index.  This happening likely means that there's a hardware
+		// error which is something we can't recover from.  The best that we can
+		// do here is to just force a flush and hope that the newly set
+		// lastFlushHash doesn't error.
+		return true, nil
+	}
+	lastFlushHeight := node.height
 	return earliestHeight > lastFlushHeight, nil
 }