Queue save() and replace() calls to prevent race conditions. [Fixes #2530506] Making multiple consecutive calls to save() or replace() would result in each call dispatching to the URL of the final call, due to a combination of HTML5 history quirks and a race condition in how dispatching was handled. The fix is to add all save() and replace() calls to a queue and ensure that we don't start processing one until the previous one (if any) has finished, thus guaranteeing that all calls will result in a dispatch to the correct URLs. We also ensure that a save() or replace() call never occurs before window.onload, since this triggers a Chrome bug that would cause a duplicate popstate event for the call that was already handled.