Changed, that Mongoid cursor is used for import, avoiding `skip()`

Iterating through mongoid collections using skip() can get really slow for large collections, as it has to traverse from the beginning of the collection (http://docs.mongodb.org/manual/reference/method/cursor.skip/).

We were seeing our import task freeze up with high I/O after importing around 2.5 million records in a 6 million record collection.

This commit alters the Mongoid import task so that it batches manually and makes the use of Mongoid's cursor to only return a small batch of results.

Closes #724
[#724] Fix incorrect Mongoid import strategy

* For collections less then :per_page, no data would be imported.
* For a collection of 1,200 items, the last 200 documents won't be imported for the default batch of 1,000

Fixes #724

Cc: @tobycox
[#724] Use modulo in Mongoid import strategy for breaking up collection

In Mongoid import strategy, use the modulo division for better semantics
of breaking up the collection into batches.

Fixes: #724