LUCENE-5092, 2nd try

In core introduce DocBlocksIterator.
Use this in FixedBitSet, in EliasFanoDocIdSet and in join module ToChild... and ToParent...
Also change BaseDocIdSetTestCase to test DocBlocksIterator.advanceToJustBefore.

This was simplified a lot by LUCENE-5441 and LUCENE-5440.