(pkg/GenDeleteSQL) duplicate calculating columns & values when generate WHERE condition

[zhangjinpeng87]

https://github.com/pingcap/tiflow/blob/master/pkg/sqlmodel/multirow.go#L86 call whereColumnsAndValues just want to compute the pre-allocate size of a slice. And https://github.com/pingcap/tiflow/blob/master/pkg/sqlmodel/multirow.go#L93 also call whereColumnsAndValues again. Please notice that whereColumnsAndValues is not a trivial function.

When the upstream TiDB is high concurrency OLTP insertion/deletion/update, it means there is not too many queries including multiple rows changes with the same timestamp for each transaction. In that case, TiCDC may call twice whereColumnsAndValues for most row changes.

We can use a fixed pre-allocation size or heuristic number instead of calculating it every row https://github.com/pingcap/tiflow/blob/master/pkg/sqlmodel/multirow.go#L87