Remove default connection if setting createInitialConnection to false after Mongoose instance created

[vkarpov15]

Summary

Right now there's always a default mongoose.connection. We added a createInitialConnection option that prevents new Mongoose instances from creating a default connection, but that doesn't apply to existing Mongoose instances, including the default one. So there's no way to really "delete" the default connection in the API.

With this PR, mongoose.set('createInitialConnection', false) will also remove the default connection, presuming it isn't connected and there are no models already on it.

Examples