Optimization: Don't create a new mosaic if the first n items are the same as the previous mosaic

[philvarner]

Currently, we create a new mosaicjson upon any change to the query. However, we don't necessarily have to, if the first n results are the same for the new query. In this case, the previous mosaicjson will have the same contents of the new one to be created, so there's no need to create the new one, so the old one can be reused until the first n items change.