Create a Lucene70DocValuesFormat

Even if it is not optimal, I think it would help to create a Lucene70DocValuesFormat now by copying the current Lucene54DocValuesFormat and including some minor changes like making the sparse case use a true iterator API as described in LUCENE-7457 (which should make it to Lucene54DocValuesFormat a well so that merging from an old codec would be efficient) as well as raising the threshold to enable sparse encoding and using nextSetBit operations when iterating bit sets, which cannot be done easily in Lucene54DocValuesFormat because we'd need to add a couple trailing bytes to make sure we can read a long at any valid index.