Fielddata: Merge ordinals APIs into BytesValues.WithOrdinals.

Mid-term we should switch from `BytesValues` to Lucene's doc values APIs, in
particular the `SortedSetDocValues` class. While `BytesValues.WithOrdinals` and
SortedSetDocValues expose the same functionality, `BytesValues.WithOrdinals`
exposes its ordinals via a different `Ordinals.Docs` object while
`SortedSetDocValues` exposes them on the same object as the one that holds the
values. This commit merges ordinals into `BytesValues.WithOrdinals` in order to
make both classes even closer.

Global ordinals were a bit tricky to migrate to I just changed them to use
Lucene's OrdinalMap that will soon (LUCENE-5767, scheduled for 4.9) have the
same optimizations as our global ordinals.