restore coinbase height check

The reason that it had to be disabled has gone away. This provides more
security by ensuring that the blocks we receive from zcashd are valid.

common/cache.go

@@ -281,8 +281,7 @@ func (c *BlockCache) Add(height int, block *walletrpc.CompactBlock) error {
 	}
 	bheight := int(block.Height)
 
-	// XXX check? TODO COINBASE-HEIGHT: restore this check after coinbase height is fixed
-	if false && bheight != height {
+	if bheight != height {
 		// This could only happen if zcashd returned the wrong
 		// block (not the height we requested).
 		Log.Fatal("cache.Add wrong height: ", bheight, " expecting: ", height)