Paginate token leader board.

Summary: Add pagination to leader board. Add key on token count in db.

Test Plan: Set page size to 1 and give tokens to two tasks.

Reviewers: epriestley, AnhNhan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5513

resources/sql/patches/20130321.token.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_token.token_count
+  ADD KEY `key_count` (tokenCount);