Issue/4795 prevent related order duplicate queries on subscriptions list table

[james-allan]

Fixes https://github.com/woocommerce/woocommerce-subscriptions/issues/4795

Description

This PR reduces the number of duplicate queries running on the WooCommerce → Subscriptions list table, decreasing them from 137 to 60. This PR focuses on two key functions:

1. Optimizing get_date_column_content()

• Previously, this function called get_time() multiple times. When rendering the Last Order Date column, each call triggered a get_related_orders() query, resulting in redundant database reads.
• The changes in this PR minimize unnecessary get_time() calls when generating date columns.

2. Improving WC_Subscription::get_related_orders()

• When get_related_orders() was called with multiple order types, it triggered separate get_related_order_ids() queries for each type.
• That would then subsequently result in a call to WCS_Related_Order_Store::instance()->get_related_order_ids() for each type and therefore a full read from the subscription meta table.
  Example:

  $subscription->get_related_orders( ..., [ 'renewal', 'switch' ] );

• Previously, this resulted in two database reads. Now, the related order caches have been updated to allow that to be 1 database read.

Summary of Changes

• Reduced unnecessary get_time() calls in get_date_column_content().
• Updated WC_Subscription::get_related_order_ids() and WCS_Related_Order_Store to allow fetching multiple related orders caches with a single database read.

How to test this PR

1. Install Query Monitor.
2. Go to WooCommerce → Subscriptions
3. Open the Query Monitor window and go to Database Queries → Duplicate Queries
4. Note the number of duplicate queries.
   • Each subscription in the list table is causing 6 calls to get_related_order_metadata()

^{137 duplicate queries.}

5. Checkout this branch and repeat the steps above.
6. The number of duplicate queries should drop by roughly 4 x the number of subscriptions you have in the list table (20).
7. There should be no difference in content or functionality on the list table.

Product impact

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

[james-allan]

I've split this PR into two parts. The date-related changes are straightforward and bring the most significant improvement, while the related order changes have a broader impact and offer less immediate benefit.

Given the difference in scope and risk between these changes, keeping them in a single PR didn’t make sense.

The two component PRs are:

• Performance - Remove unnecessary get_time() calls to reduce redundant get_last_order() requests on admin Subscriptions list table #789
• Performance - Reduce unnecessary database reads in get_related_orders() when fetching multiple order types #790