Simplify account sync start logic.

Summary:
This commit simplifies our sync start logic.

Our whole sync start/stop/reporting deal is due for an overhaul
but till that happens, this makes the sync start/stop +
admin dash interaction easier to understand and faster to debug.

Other changes included:
1. Remove sync_type - never used.
2. Always update sync_start_time - this is what the dash displays.
If we want the original sync_start time, we can go look at the db.

Test Plan:
Started a sync.
While it ran, authed another account.
While both ran, stopped the first.
Restarted the first.

Please do stare at it carefully nonetheless.

Reviewers: emfree, charles

Reviewed By: charles

Differential Revision: https://review.inboxapp.com/D591