[schedule/work-stealing] Make work-stealing be a configurable option.

Summary: add a configuration option to control when machines should work-steal accounts.

Test Plan: read the code

Reviewers: kav-ya

Reviewed By: kav-ya

Subscribers: emfree

Differential Revision: https://review.inboxapp.com/D610

Conflicts:
	inbox/mailsync/service.py
[stop-sync] Re-work the logic around stopping syncs to make sure that the events and contacts syncs get stopped too.

Summary:
Previously stopping a sync would result in the events / contacts syncs continuing to run. Re-work the logic so that
we actually stop these syncs too (and remove some redundant checking).

Test Plan: review the code, start / stop syncs and make sure contacts / events sync stop too.

Reviewers: kav-ya

Reviewed By: kav-ya

Subscribers: emfree

Differential Revision: https://review.inboxapp.com/D608

Conflicts:
	inbox/mailsync/service.py