Summary:
Using a TLS random instance for skiplist makes it smaller
(useful for hash_skiplist_rep) and prepares skiplist for concurrent
adds.  This diff also modifies the branching factor math to avoid an
unnecessary division.

This diff has the effect of changing the sequence of skip list node
height choices made by tests, so it has the potential to cause unit
test failures for tests that implicitly rely on the exact structure
of the skip list.  Tests that try to exactly trigger a compaction are
likely suspects for this problem (these tests have always been brittle to
changes in the skiplist details).  I've minimizes this risk by reseeding
the main thread's Random at the beginning of each test, increasing the
universal compaction size_ratio limit from 101% to 105% for some tests,
and verifying that the tests pass many times.

Test Plan: for i in `seq 0 9`; do make check; done

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D50439