Replace `wp_count_posts( 'shop_subscription' )` with new data store function to work with HPOS

[mattallan]

Fixes #382

Description

In #372 we introduced a new data store function that returns an array of subscription statuses and the number of subscriptions on the store with that status:

WC_Data_Store::load( 'subscription' )->get_subscriptions_count_by_status()

In this PR, I'm updating all uses of wp_count_posts() with this new data store function.

Notes

• The wcs_is_large_site() was also counting the number of orders on a store but there's currently not HPOS equivalent function for this so for the time being I've opted into just leaving this count out of the condition when HPOS is enabled.
• There's a wp_count_posts() calls in the repair script files that I didn't bother updating in this PR because it's a repair script specifically for stores upgrading from 2.0.0 to 2.0.2

How to test this PR

To test the Admin System Status changes:

1. Enable HPOS and purchase a handful of subscriptions
2. Change some statuses of the subscriptions
3. View WooCommerce > Status
4. Scroll down to Subscriptions section and look at the Subscription Statuses: row.
5. Disable HPOS and purchase some more subscriptions to test with HPOS disabled.

For the rest of the changes I ran the following snippet in WP Console and tested with HPOS enabled and disabled:

echo 'WC_Subscriptions::get_subscription_status_counts(): ' . var_export( WC_Subscriptions::get_subscription_status_counts(), true ) . "\n";

echo "\n\n";
delete_option( 'wcs_is_large_site' );

echo 'wcs_is_large_site(): ' . var_export( wcs_is_large_site(), true );

echo "\n\n";

echo 'WCS_Admin_System_Status::get_subscription_statuses(): ' . var_export( WCS_Admin_System_Status::get_subscription_statuses(), true );

Product impact

• [x] Added changelog entry (or does not apply)
• [x] Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [x] Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• [ ] Added deprecated functions, hooks or classes to the spreadsheet