Support useUnifiedTopology in connection.openUri

[AndrewBarba]

Feature Request:

Add support for useUnifiedTopology in Connection.openUri

Explanation:

Mongoose 5.7 upgraded to MongoDB v3.3 which supports the new useUnifiedTopology option. Currently Mongoose supports this in createConnnection but does not support it in Connection.openUri. We use openUri to delay the actual connecting process until something uses it, thus we do not pass any options into createConnection when our app starts up.

[AndrewBarba]

This might actually be supported as is, and is just missing from the openUri docs

[vkarpov15]

Looks like this issue was just in the docs, should be fixed in 5.7.1, which we will release within the next couple days 👍