Support window functions for TiDB

[hanfei1991]

[shenli]

MySQL window functions: https://dev.mysql.com/doc/refman/8.0/en/window-functions.html

[morgo]

Window functions are being added in this project: https://github.com/pingcap/tidb/projects/18

To reduce duplication, I am going to close this issue now.