Fix MySQL visibility indexes with close_time column

[rodrigozhou]

What changed?
Cast max datetime string to datetime type in MySQL COALESCE calls.
Refactor getCoalesceCloseTimeExpr to be part of pluginConverter interface.

Why?
MySQL coalesce with datetime and string results in a string, which can cause incorrect comparisons with datetime (eg: times 12:12:12 is different than 12:12:12.000).
So, adding a CAST to max datetime so it's handled correctly as datetime type column.
This also fixes flaky tests in TestMySQL8VisibilityPersistenceSuite

How did you test it?
Hard coded a time with leading zeros, and tests are now passing.

Potential risks
No risks.

Is hotfix candidate?
No.