This class is starting to get larger and larger as we add more helpers, some of which are shared between the read path and write path for persistent samples.

If we eventually support multiple storage formats (chained documents and columnar) then it will be helpful to restructure this class such that we can eventually have subclasses inherit the shared methods and provide their own overrides for format-specific functions.

Currently persistent_sample_loader.h/.cpp also include a number of constants (eg. kSamplesCollectionName, kPersistentSampleSchemaVersion) and helpers that are not members of the PersistentSampleLoader class. These are likely better placed in a shared util file of some kind rather than with the class itself.