Summary:
This diff is 2/3 in a sequence that introduces a skip list optimized
for a key that is a freshly-allocated const char*.  The change is broken
into pieces to make it easier to review.  This piece removes the Key
template type, introduces the AllocateKey interface, and changes the
unit test from using uint64_t as the Key type to using pointers to an 8
byte blob.

Test Plan: unit test

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D51285