Add Skip List implementation

[Boiarshinov]

It seems that there is no Skip list implementation yet.
According to Wiki Skip list is a probabilistic data structure that allows O(log n) search complexity as well as O(log n) insertion complexity within an ordered sequence of n elements.
It's a very interesting data structure used by many DBMS: Apache Cassandra, Redis, WiredTiger and others

[Boiarshinov]

I had prepared PR with Skip List realisation: #3154