VersionManager.as_of(): avoid using OR in query

[tbazadaykin]

When using as_of() it using OR operator into query to add current record into result.

>>>print MyMode.objects.as_of(timezone.now()).query
SELECT `mymodel`.`id`, `mymodel`.`identity`, `mymodel`.`version_start_date`, `mymodel`.`version_end_date`, `mymodel`.`version_birth_date` FROM `mymodel` WHERE ((`mymodel`.`version_end_date` > 2018-03-20 14:30:18 OR `mymodel`.`version_end_date` IS NULL) AND `mymodel`.`version_start_date` <= 2018-03-20 14:30:18)

If set version_end_date default value to datetime.max (or other hard-to-reach value) it is possible to avoid using OR. This will lead to a decrease of query cost.

SELECT `mymodel`.`id`, `mymodel`.`identity`, `mymodel`.`version_start_date`, `mymodel`.`version_end_date`, `mymodel`.`version_birth_date` FROM `mymodel` WHERE (`mymodel`.`version_end_date` > 2018-03-20 14:30:18 AND `mymodel`.`version_start_date` <= 2018-03-20 14:30:18)

[maennel]

Hey @tbazadaykin, thanks for coming up with this. I see a bit of a trade-off in identifying current versions - but this may be less of a problem than it seems, if the column is properly indexed.
I will not have time to go deeper into this during the upcoming couple of weeks, but will try to get back to this point sometime later.