Remove old and unused skip-list.* from lib.
Add generic skip list functionality.
Remove unneeded include of old skip-list.h
Fix compilation of new skip-list.c
Remove old arangod/SkipLists/skiplist.* from Makefile.
Follow coding style guidelines a bit more. Add nrUsed count.
Add a cmpdata entry for comparisons.
Get rid of the distinction between unique and multi skiplist indices.

This is the change outside the actual skiplist index code.
Add keys to the generic skip list implementation.
Export entry count for generic skip lists.
Rename method name to TRI_SkipListGetNrUsed.
Add a sparse argument to the creation of a skiplist index.
Snapshot, arangod/SkipLists/skiplistIndex.* is unfinished.

--> Does not compile.
Still need to rework the iterator code.
Snapshot for transport, still not compiling.
Compiles again, untested.
Fix first bug.
Remove old skiplist code.
Fix bug with initialisation of _cursor.
Fix another bug in skiplist indices.
Sort out return type of NextIterator for skiplist indices.