Summary:
This diff is 1/3 in a sequence that introduces a skip list optimized for
a key that is a freshly-allocated const char*.  The diff is broken into
pieces to make it easier to review.  This piece only introduces the new
type by copying the existing SkipList, with mechanical naming changes
and reformatting.

Test Plan: new unit test

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D51279