ResultRetriever should batch clusterings/rows during SAI post-filtering reads

SAI currently creates and executes a SinglePartitionReadCommand for every PrimaryKey the index produces to read the corresponding row for post-filtering. Informed by the limits present in the read command itself, it should be possible to batch those reads w/ a ClusteringIndexNamesFilter in many fewer SinglePartitionReadCommands. When we have a handful of matches in a large partition, this seems like would involve many fewer seeks, less unnecessary object creation, etc.