Make our syncback algorithm a fair bit fairer
Make our syncback algorithm a fair bit fairer

Summary:
Our current syncback algorithm is very simple --- we simply execute the ActionLog entries in order. This breaks down when somebody decides to queue 100k actions.

This diff changes the algorithm to be a bit fairer: at each iteration we execute an action for a `NUM_PARALLEL_ACCOUNTS` different accounts. This means that every account gets a chance to execute their actions.

Test Plan: Tested manually, will be more serious on staging.

Reviewers: spang

Reviewed By: spang

Subscribers: drew, jenkins

Differential Revision: https://phab.nylas.com/D3038

jobscore/sync-engine,closeio/nylas,nylas/sync-engine,jobscore/sync-engine,nylas/sync-engine,jobscore/sync-engine,closeio/nylas,closeio/nylas,nylas/sync-engine,jobscore/sync-engine,closeio/nylas,nylas/sync-engine