Store documents in a skiplist index even if some indexed attrs are empty.
Adjust test to new non-sparse skiplists.
Introduce a boolean switch _sparse in TRI_index_t.

At this stage it is always set to false for all indexes.
However, TRI_InitIndex already has an additional argument for this.
Correct skiplist fix for empty attribute values.
Fix a bug with the level management of the useSortRule.
Make skiplist index insertion aware of the sparse flag.

Note that so far this flag is always false.
Remove "old" sparse flag for skiplist indexes.