Move product sync to web worker, fix for Firefox

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-pos/trunk@925193 b8457f37-d9ea-0310-8a92-e5e31aec5664
fix worker.js path

git-svn-id: https://plugins.svn.wordpress.org/woocommerce-pos/trunk@925196 b8457f37-d9ea-0310-8a92-e5e31aec5664