Summary:
Fixes bug where contact ranking was not being fetched, and refactors the refreshing
of contact ranks. Moves periodic refreshing of the database-stored ranks to the sync
workers so it occurs in the background, once per account. Refactors JSON cache code
accordingly.
Test Plan: manual
Reviewers: evan, bengotow
Reviewed By: bengotow
Differential Revision: https://phab.nylas.com/D2137
