WooCommerce Lightspeed POS Integration 2.16.6

* Tweak - Optimization improvements utilizing wp_object_cache (thank you Xavier)
* Tweak - Moves the poller to Action Scheduler, allowing manual runs of syncing as well as reduced 429 errors
* Fix - Removes WCLSI_Cache_Lock class, should fix any database duplicate errors
* Fix - Deletes categories from the wp_wclsi_item_categories table when the corresponding categories in WooCommerce are deleted
* Fix - Fixes an issue during Lightspeed authentication where the tokens do not get parsed correctly

* Fix - Fixes a bug where products were still importing even though the auto-import setting was set to "Do nothing"
* Fix - Sets sslverify to true by default, fixes the "Certificate verification disabled (sslverify=false)" error
* Fix - Fixes a "Duplicate Key" error around the some cache locking logic

* New - Moves "check_for_new_ls_prods()" to an action scheduler job, the runs every 30 seconds. Also allows users to trigger the job manually.
* Fix - Fixes some 429 errors around the `woocommerce_update_product` action hook. This gets called multiple times and triggers a `update` Lightspeed API call erroneously
* Fix - Fixes "Duplicate Key" errors resulting from a bug in the new WCLSI_Cache_Lock class
* Fix - Fixes an error around the refresh_access_token() function where sometimes it would return a WP_Error object