Consider putting skip stages below fetch stages

Suppose we have a query for which the following conditions hold:

1. The query predicates can all be answered using an index.
2. The query is not covered (i.e. it requires a fetch stage) because there is no projection.
3. There is a large skip value.

Currently, we will fetch documents for all the skipped results. This means unnecessary IO / unnecessarily large nscannedObjects. As an optimization, we could avoid the extra fetching by putting the skip stage below the fetch.

As an example, say we have documents of the form {a: <int>, b: <int>, c: <int>, x: <int>} and an index {a: 1, b: 1, c: 1}. The following query

db.coll.find({a: 3, b: 4}).sort({c: 1}).skip(10000);

can use the index to answer both query predicates and to do the sort. However, the query is not covered: we need to fetch documents in order to get the '_id' and 'x' fields. In this case it is possible to put the skip stage below the fetch in order to avoid fetching the 10,000 skipped documents.