Handles a terms dict, but decouples all details of doc/freqs/positions reading to an instance of {@link PostingsReaderBase}. This class is reusable for codecs that use a different format for docs/freqs/positions (though codecs are also free to make their own terms dict impl).
This class also interacts with an instance of {@link TermsIndexReaderBase}, to abstract away the specific implementation of the terms dict index.
@lucene.experimental