MultiLevelSkipListWriter

This abstract class writes skip lists with multiple levels.

Example for skipInterval = 3:
                                                     c            (skip level 2)
                 c                 c                 c            (skip level 1)
     x     x     x     x     x     x     x     x     x     x      (skip level 0)
d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d  (posting list)
     3     6     9     12    15    18    21    24    27    30     (df)

d - document
x - skip data
c - skip data with child pointer

Skip level i contains every skipInterval-th entry from skip level i-1.
Therefore the number of entries on level i is: floor(df / ((skipInterval ^ (i + 1))).

Each skip entry on a level `i>0` contains a pointer to the corresponding skip entry in list i-1.
This guarantees a logarithmic amount of skips to find the target document.

While this class takes care of writing the different skip levels,
subclasses must define the actual format of the skip data.