Tweaks to FeatureIndex::findMulti

This should make the code slightly leaner and easier to understand

* filterResults() only once on full resultset instead of 2 seperate times
* more comments
* only do $keyToQuery stuff when needed
* only pass cache-resolved $queries to expandCacheResult

Change-Id: I7fd217823d62edae46237f368fba8edac86ffa13
Merge "Tweaks to FeatureIndex::findMulti"