Fix mongodb populate falling down performances for big collections

Using skip to retrieve slices of data is not performant when the amount of documents skipped is important. It decreases the performances of the command `fos:elastica:populate` for each retrieved slice until it crashes (30s timeout).

http://docs.mongodb.org/manual/reference/method/cursor.skip/

To fix this problem, a solution is to retrieve a slice from the last object of the previous slice. Mongo can then use an index to find the slice instead of browsing all the collection.