Summary: While working on separating send out of the sync loop, I realized sync tasks could use some cleanup to be more consistent with how we implemented syncback tasks. I reorganized and renamed things a little bit. This will also help us move in the direction of the scheduler implementation under which everything is a task.
Test Plan: manual
Reviewers: evan
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D3660