* Tweak: Clear cache when order updated.

* Tweak: Clear database cache faster when API Resource no longer exists.

Tweak: On Order edit update of Access Expires and/or Activations, process API cache refresh in task queue rather than immediately.

Tweak: Delete API Key activations when WooCommerce Subscription transitions to on-hold status.

 * Feature: Added a product_list API endpoint which returns a unique list of product data such as Titles and Product IDs with no authentication required.

 * Fix: get_human_time_diff() method returned slightly incorrect time for date.

 * Fix: Order edit API Resources meta box > Activation Expires field displays only a date now rather than friendly messages such as "4 days", as friendly messages broke updates using a value that was not recognized as valid.

 * Fix: Order edit API Resources meta box > Activation Expires help tip was displayed for WooCommerce Subscription when it should be hidden.

 * Fix: Order edit API Resources meta box > Activation Expires field was updated when the Update button was clicked without changing the value manually. No update should have occurred.

 * Tweak: Prevent Order edit API Resources meta box > Activation Expires field from being set to value less than current value.

 * Fix: When the Product edit > API > Activation Limit field is updated both the activations_purchased and activations_purchased_total values are now calculated with regards to quantity and refunds for all API Resources containing the Product.

 * Fix: When the Order edit API Resources meta box > Activation Limit field is updated both the activations_purchased and activations_purchased_total values are now calculated with regards to quantity and refunds for the API Resource containing the Product.

 * Tweak: Added $source parameter to error and info log methods to allow for custom log file names.

 * Tweak: Added visual calculation aid for Order edit API Resources meta box > Activation Limit Total.

 * Tweak: Other visual and tip improvements.