Set idle connection pool size to 256.

No substantial benefit was observed to setting it higher, based on
local testing.

go/src/hello/hello.go

@@ -35,7 +35,7 @@ const (
 	WorldUpdate        = "UPDATE World SET randomNumber = ? where id = ?"
 	FortuneSelect      = "SELECT id, message FROM Fortune;"
 	WorldRowCount      = 10000
-	MaxConnectionCount = 5000
+	MaxConnectionCount = 256
 )
 
 var (