Optimize getPrevNodeFromNode.

The recent pruning code made the parent hash for a node available directly
as a field in the node.  Make use of this in the getPrevNodeFromNode
function to avoid having to load the full block data associated with the
node to get the parent hash.

chain.go

@@ -359,17 +359,9 @@ func (b *BlockChain) getPrevNodeFromNode(node *blockNode) (*blockNode, error) {
 		return nil, nil
 	}
 
-	// Load the actual block for this block node from the db to ascertain
-	// the previous hash.
-	block, err := b.db.FetchBlockBySha(node.hash)
-	if err != nil {
-		return nil, err
-	}
-
 	// Dynamically load the previous block from the block database, create
 	// a new block node for it, and update the memory chain accordingly.
-	prevHash := &block.MsgBlock().Header.PrevBlock
-	prevBlockNode, err := b.loadBlockNode(prevHash)
+	prevBlockNode, err := b.loadBlockNode(node.parentHash)
 	if err != nil {
 		return nil, err
 	}