Override the default MySQL query limit when counting orders

[stevegrunwell]

OrderLimiter::count_qualifying_orders() relies on wc_get_orders(), which is several layers of abstraction over WP_Query.

Currently, if we analyze the query, it looks something like this:

SELECT wptests_posts.ID
FROM wptests_posts
WHERE 1=1
AND ( wptests_posts.post_date_gmt >= '2020-04-17 00:00:00' )
AND wptests_posts.post_type = 'shop_order'
AND (
    (
           wptests_posts.post_status = 'wc-pending'
        OR wptests_posts.post_status = 'wc-processing'
        OR wptests_posts.post_status = 'wc-on-hold'
        OR wptests_posts.post_status = 'wc-completed'
        OR wptests_posts.post_status = 'wc-cancelled'
        OR wptests_posts.post_status = 'wc-refunded'
        OR wptests_posts.post_status = 'wc-failed'
    )
)
ORDER BY wptests_posts.post_date DESC LIMIT 0, 10

Unfortunately, that last line is the problem: we're limiting the qualifying orders to 10, which is the default LIMIT value for WP_Query.

This PR overrides that default, setting it to either the limit set by the store or 1000 (whichever is higher); this ensures that we're not running a -1 limit on a store that could potentially have a very large threshold for orders (e.g. "oh, we only want to accept 1M orders/week").

Eventually, it may be worth rewriting this function to use an explicit SQL COUNT(*) query (thus eliminating limit concerns), but we want to ensure we do that in a way that maintains compatibility with WooCommerce's CRUD APIs.

This should also resolve the two open support tickets on WordPress.org:

1. https://wordpress.org/support/topic/limit-orders-did-not-work/
2. https://wordpress.org/support/topic/orders-have-exceeded-the-limit/