Support push down sysdate() to TiKV

[Yisaer]

Enhancement

Currently, sysdate() couldn't been folded during optimization. In this issue, we wanted to push down sysdate() in following rule:

1. support global variable sysdate-is-now, default false
2. If sysdate-is-now is true, we should fold sysdate during optimization
3. if sysdate-is-now is false, we should push down sysdate into TiKV

ref https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_sysdate