WooCommerce Lightspeed POS Integration 2.16.0

* Fix - Fixes erroneous LS update callbacks that may have been triggered via auto polling and other product update pathways
* Fix - Fixes "PHP Notice: Undefined property: stdClass:httpCode in class-wclsi-init-settings.php" due to token refresh logic
* Tweak - Lowers chances of 429 throttling errors triggered via the "Manual Update via Lightspeed" button for variable products

* 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