Set best chain during GenerateInitialIndex.

This commit modifies the GenerateInitialIndex function to update the best
chain with each node as it is loaded.  This allows the best chain to be
set immediately upon generating the initial index which is a slight
performance optimization.

chain.go

@@ -379,10 +379,13 @@ func (b *BlockChain) GenerateInitialIndex() error {
 			return err
 		}
 
-		_, err = b.loadBlockNode(hash)
+		node, err := b.loadBlockNode(hash)
 		if err != nil {
 			return err
 		}
+
+		// This node is now the end of the best chain.
+		b.bestChain = node
 	}
 
 	return nil