Upgrade to MongoDB Driver 4.x

[vkarpov15]

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[vkarpov15]

Notes on some breaking changes:

1. The overwrite option to update(), updateOne(), etc. is no longer supported. You need to use replaceOne() instead
2. Connection#host, Connection#port, Connection#name will not be set until after connection is established.
3. MongoDB driver 4.x requires Node.js 10 or higher.

[vkarpov15]

4. MongoDB driver no longer supports bufferMaxEntries
5. The result of updateOne() and updateMany() operations no longer has ok, n, or nModified properties. These properties are replaced with acknowledged, matchedCount, and modifiedCount
6. The result of deleteOne() and deleteMany() operations no longer has an n property, it has been renamed deletedCount

[vkarpov15]

7. No more geoSearch() or geoHaystackSearch() re: https://docs.mongodb.com/manual/reference/command/geoSearch/
8. poolSize option is gone, replaced by minPoolSize and maxPoolSize