Implement additional operations and dynamic resizing on skiplists

[polytypic]

PR #99 added a lock-free skiplist. As mentioned in the PR, there is still a lot of room to improve on it:

• The maximum node height is not adjusted dynamically, which requires one to have a good idea of the number of bindings in the skiplist for performance.
• Skiplist can support operations like finding/removing the binding with the minimum / maximum key, listing bindings in (ascending) order of their keys, finding bindings with keys in a given range (min, max) of keys, finding the binding with the smallest key that is greater or equal to search key, ... all the things you'd expect from an ordered container.