fix: add max limit of 5 consecutive scheduled syncs

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Adds a mechanism to the schedule_sync method so that consecutive scheduled syncs for a single contact will only happen up to a max of five consecutive times. This is to avoid an infinite loop if a contact is scheduled to retry the sync after a failure, but the sync keeps failing.

The counter is reset upon a successful sync to the ESP.

How to test the changes in this Pull Request:

Follow instructions in Automattic/newspack-newsletters#1729.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[dkoo]

Since we're coming right up against the holidays at this point, we're going to hold this change for when the team is back at full strength.