MongoStore deprecated methods

[Pfahlf]

While running with the latest version of Mongoose, I get the following warnings:

(node:31651) DeprecationWarning: collection.remove is deprecated. Use deleteOne, deleteMany, or bulkWrite instead.
(node:31651) DeprecationWarning: collection.findAndModify is deprecated. Use findOneAndUpdate, findOneAndReplace or findOneAndDelete instead.
(node:31651) DeprecationWarning: Mongoose: `findOneAndUpdate()` and `findOneAndDelete()` without the `useFindAndModify` option set to false are deprecated. 

Moreover, is good to put into the examples the following options for the mongoose connection:

useCreateIndex: true

in order to avoid the deprecation warning:

DeprecationWarning: collection.ensureIndex is deprecated. Use createIndexes instead.

Is there any intent to fix these warnings?

[julienwilmet]

Feel free to make a PR, it's on our todo though so when we have free time we'll check this!