This patch merges the old DocsEnum and DocsAndPositionsEnum into a new PostingsEnum class (which is basically the old DaPE class), with DocsEnum extending it as a convenience class returning empty values for positions, offsets and payloads.
TermsEnum.docs() methods are renamed to TermsEnum.postings().
The old docs() and docsAndPositions() methods can be added back to keep backwards compatibility.
Next up: some basic re-use tests. I think we should be able to assert that things aren't reused when we have different postings requested for all postings formats, and check specific cases for those formats where re-use is actually implemented.