fix: restore auto direct connection behavior

[emadum]

Description

We removed automatic direct connection for the unified topology in the 3.6.3 release of the driver. This change was preparatory for the 4.0 version of the driver, where we'll always perform automatic discovery. This PR restores automatic direct connection when connecting to a single host without a specified replicaSet and without directConnection: false.

In #2557 this was justified because useUnifiedTopology: true is an opt-in feature, but we've had a number of users file tickets about this change, and it's probably more appropriate to delay it until the major release of the driver, along with a call-out/explanation in the migration guide, rather than putting it in a patch release. That said, I'm open to closing this PR if we decide the original reasoning was appropriate.

NODE-2966

What changed?

Are there any files to ignore?

[durran]

@emadum What do you think about adding a test for the specific scenario? Even with the functionality going away I think it would be beneficial not to have a regression later in the 3.6 branch. I was expecting a test where directConnection and replSet are both not supplied when connecting directly to a replica set node.

[emadum]

@durran Adding a test definitely seems reasonable! I'll do so once it's confirmed we all agree about reverting this change.

[nbbeeken]

I agree with the change, should we consider a deprecation warning of sorts? as long as it can be hushed I think its a good place to notify users.

[emadum]

I agree with the change, should we consider a deprecation warning of sorts? as long as it can be hushed I think its a good place to notify users.

Just to have a record of our conversation, we aren't going to emit a deprecation warning because it would also be emitted whenever connecting to a standalone MongoDB server. However we should definitely make sure to explain this change in the 4.0 migration guide.