UnifiedHighlighter should use single OffsetEnum rather than List<OffsetEnum>

The UnifiedHighlighter deals with several different aspects of highlighting: finding highlight offsets, breaking content up into snippets, and passage scoring.  It would be nice to split this up so that consumers can use them separately.

As a first step, I'd like to change the API of FieldOffsetStrategy to return a single unified OffsetsEnum, rather than a collection of them.  This will make it easier to expose the OffsetsEnum of a document directly from the highlighter, bypassing snippet extraction and scoring.