Use singleton MongoDB client for uploader

[jpahm]

Currently, the uploader gets a MongoDB client from the connectDB function in uploader/database.go. However, this client isn't a singleton and could easily lead to multiple mongo clients being created accidentally in the future.

We should instead wrap the mongo client in a singleton instance the same way it is done for the API.

[Yogeb-tech]

May I take this one?

[mikehquan19]

Closed via #89