Add openUri() and useMongooseUri option, deprecate open() and openSet()

[vkarpov15]

#5304

Mongoose's default connection logic is deprecated by this PR. Please opt in to the new connection logic using the useMongoClient option (or use openUri() if you're using open() or openSet()), but make sure you test your connections first if you're upgrading an existing codebase!

    // Using `mongoose.connect`...
    var promise = mongoose.connect('mongodb://localhost/myapp', { useMongoClient: true, /* other options */ });
    // Or `createConnection`
    var promise = mongoose.createConnection('mongodb://localhost/myapp', { useMongoClient: true, /* other options */ });
    // Or, if you already have a connection
    connection.openUri('mongodb://localhost/myapp', { /* options */ });

This deprecation is because the MongoDB driver has deprecated an API that is critical to mongoose's connection logic to support MongoDB 3.6, see #5304 for details.

[sobafuchs]

lgtm, should we change the connection logic in the tests to use useMongooseUri or should we just allow those errors to clutter up the test output?

[vkarpov15]

Good suggestions, thanks. Will use useMongoClient instead.