Recurring Payments: get rid of the CPT caching

Recurring payments stores product data as both CPT and an entry in the WPCOM billing engine
These requires syncing and sometimes cause issues.
Additionally, it requires fair bit of complexity to get to the proper product, etc.

For example, the ID presented on the site is the ID of the CPT from where an id in the billing system is pulled and displayed in the checkout window.

We should:
a) Introduce direct usage of the billing system ids
b) transition everything to these ids, including existing buttons on existing sites
c) phase out the legacy CPT system, only using the billing tables ids

No files or tests are named. Start by tracing recurring payments from the CPT product ID to the WPCOM billing engine ID and checkout display; map how existing buttons store and resolve these identifiers. Done means recurring payments use billing-system IDs directly, existing buttons are transitioned, and the legacy CPT path is phased out.

Written by the indexing model from the issue text.