WooCommerce PDF Product Vouchers 3.12.3

* Fix - Fix a possible race condition when performing background processing health checks
* Fix - Account for possible false negatives when testing loopback connections in certain environments

* Feature - Offload PDF generation from checkout to a background job to vastly improve checkout performance, especially if multiple vouchers are purchased
* Tweak - Show an admin notice if voucher generation not supported
* Tweak - Save voucher activation date as postmeta
* Fix - Errors in v3.0 upgrade if voucher image attachments are not found or redemption amount is invalid
* Fix - Add line breaks in plain text email to voucher recipients for more readability
* Fix - Bypass tax calculation when taxes are completely disabled for the store
* Fix - Address an issue where voucher PDF generation failed on some PHP versions due to whitespaces between HTML tags
* Misc - Update DomPDF to 0.8.1

* Tweak - Improve compatibility with Print Invoices / Packing Lists to show voucher information
* Fix - Check if other plugins are loading the Dompdf library used by PDF Vouchers to avoid incurring into PHP "Cannot declare class" fatal errors