Start to reintroduce backpointers in skiplist.

Also need an non-nullptr end node now.
C++-ize skip lists and introduce a backward link.
Merge branch 'aql2' into skiplistback
Fix a bug introduced in the skiplist code when C++izing it.
Unuse index-iterator.h
Merge branch 'skiplistback' into aql2
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2