Improve slow query for legacy subscriptions on WC status tools page by caching the data

[Mayisha]

Fixes #3755

Changes proposed in this Pull Request:

To avoid frequent unnecessary DB calls on WooCommerce > Status > Tools page, we are caching the number of subscriptions having the legacy Stripe Sepa as the payment method.

This is not an issue for the sites with a small number of orders/subscriptions but it will improve performance for the sites having a large number of orders. The site used in #3755 has ~1 million orders and this query takes ~2 sec each time.

Testing instructions

• Enable the subscriptions plugin.
• Create subscription products and purchase a few subscriptions as a shopper if you already don't have any.
• Go to WooCommerce > Status > Tools page and open the query monitor.
• As you don't have the cached data by this time, you should see the following DB query.

• Now refresh the page and confirm that the above DB query is not executed this time.
• Check your DB and confirm that you have the woocommerce_stripe_subscriptions_with_legacy_sepa transient set.

[diegocurbelo]

LGTM 🚢

[diegocurbelo]

LGTM 🚢