Follow-up from SERVER-37715

Supported scenarios, given a compatible index:

• $unwind+$match+$group since the filter applies to every element in the array, corresponding to index keys
• $match+$unwind+$group on a non-multikey index (though I imagine it's uncommon to have queries like this)

This change will also make eligible $unwind+$group queries use multiplanning, so we should make sure to add relevant tests