WooCommerce Lightspeed POS Integration 2.16.0

* 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

* New - Removes the need to have allow_url_fopen = on, moves image downloads to native wp_remote_get() function
* Fix - Fixes some issues around Javascript setTimeout() functions that were introduced in v1.6.2. Should help reduce 429 errors.

* New - Adds a "Stock Quantity (non-manual updates, e.g. orders)" selective sync setting specific to WooCommerce Orders, if checked it will sync inventory to Lightspeed after a WooCommerce order is completed
* Fix - Fixes a sync bug where image changes in Lightspeed were not being synced over to WooCommerce
* Fix - Fixes a sync bug that could have locked the poller from syncing for long periods of time (possibly hours)
* Fix - Fixes a sync bug where inventory was inadvertently being synced over to Lightspeed