fix persistent connections

[timgraham]

Without autocommit set, BaseDatabaseWrapper.close_if_unusable_or_obsolete() short-circuits early and always closes the connection after each request.

[aclark4life]

@timgraham Setting autocommit does not mean that autocommit occurs, correct?

[timgraham]

Query execution uses Django and MongoDB’s default behavior of autocommit mode. Each query is immediately committed to the database.

Part of the purpose of set_autocommit() is to allow the user to disable transaction management. As far as I know, there is no way to disable transaction management in MongoDB, and this method shouldn't need any adjustments.

[aclark4life]

I noticed you didn't use the "backend specific" _set_autocommit(). Was that intentional and if so is that because we don't need what's in set_autocommit() ? Assuming _set_autocommit works the same (because it's called by set_autocommit) should we use that instead, because it is potentially less confusing?

[timgraham]

Since the backend has no transaction support, overriding set_autocommit() avoids a lot of uneeded things. It might be that adding support for transactions requires overriding _set_autocommit() instead (it would just be an empty method).