WooCommerce Lightspeed POS Integration 2.16.0

* Fix - Fixes a bug that was introduced in v1.5.9 where new LS products were not being automatically loaded
* Fix - Fixes a bug where attributes for variable/matrix products were not being synced over to WooCommerce correctly
* Tweak - Changes the syncing algorithm to look back an extra 30 seconds allow some overlap in updates made in LS

* Adds a "Sync" checkbox on the import page to allow to also sync all items after they import
* Adds a "Ignore Archived Lightspeed Products" setting; if enabled, will ignore archived items in LS when items are loaded in WooCommerce
* Changes the polling interval from every 5 minutes to every 5 seconds to keep products more up to date
* Reduces "429 - Too many requests" error frequency
* More informative item counts - displays both simple and matrix item counts under the search box in the import page
* Stock status bugfix - sets or removes "outofstock" term under the "product_visibility" taxonomy
* Sync php warning bugfix - does not enable the item poller if connection to LS is not set

* New - Re-linking a product will now match on customSku as well as systemSku
* New - New Item Poller class should make polling for Lightspeed products more efficient
* New - Item Poller class will keep items in the import table up to date (even though they have not been imported yet)
* Fix - Attributes with values of '0' should now be imported correctly
* Fix - Fixes incorrect SQL syntax for auto-loading products
* Tweak - Better testing coverage, minor bug fixes, and DRYing up code