Athens: DropDups is silently ignored after MongoDB >2.8

[chriscoffee]

After some discussion in #athens we figured out that dropDupes is no longer available after 3.0 for createIndex(), ensureIndex(), and createIndexes.

The latest stable version of MongoDB is 4.0.

Should we consider dropping this or continue to support it to allow some backwards compatibility? And should it be true?

For those wanting to know what DropDups does:
DropDups bool // Drop documents with the same index key as a previously indexed one

[michalpristas]

i think with other storage implementations we return error with module already exists we should do the same here