Change field data apis to use BaseCompositeReader instead of DirectoryReader

[martijnvg]

Because we use DirectoryReader we can let the lucene test framework randomily wrap the readers in our aggregation unit tests:
5bbfb6c#diff-ab07a3e29560885c391583295d69c3d1R101

If we would change all field data and field data cache signatures to use BaseCompositeReader instead of DirectoryReader we would be able to randomily wrap readers in these tests.

[jpountz]

I think it was done this way because directory readers guarantee that they remain open for a long time, while index reader wrappers may be short-lived, which can cause performance bugs.

[javanna]

@martijnvg is this still valid?

[martijnvg]

The problem only occurs if Lucene test framework wraps the reader with specific readers (e.g. ParallelCompositeReader) The reason I like to use LuceneTestCase#wrapReader(...) is because it randomly wraps the reader in an AssertingDirectoryReader, which can detect misuse of the Lucene internal apis. I think as a compromise we can instead add our own wrapReader(...) method that only randomly wraps the reader in an AssertingDirectoryReader.