MongoClient lifetime scope

Hey
I found that a MongoClient is created per each HTTP request. MongoDB strongly advises on a single instance since they [don't dispose of resources correctly](https://jira.mongodb.org/browse/CSHARP-3431).
More details [here](https://www.mongodb.com/community/forums/t/best-practice-for-singleton-vs-multiple-mongoclients-when-you-need-to-authenticate-with-different-users/125935/6).