feat(connections): Adds default database connection string support (#8354)

[zachazar]

Summary
This solves #8354. It appears that previously SRV connection strings didn't allow for default databases to be in the uri. I'm not sure if that changed recently, but when connecting to our Atlas clusters we are able to specify the default database in the string.

This PR changes the logic when assigning the database name (as outlined in #8354) to look for options.dbName, then look for the default database in the connection string, then fallback to the auth db.

Examples

mongoose.connect('mongodb+srv://<username>:<password>@cluster0.mongodb.net/defaultDatabase?w=majority')

// => results in a connection with `db.name === defaultDatabase`

[vkarpov15]

Thanks!