After the first request comes in (subscriptions for global endpoint and clusters for regionalized endpoints), calculate how many more pages are needed and add them to the queue immediately.   This allows the browser to make page requests concurrently.

Ideally, this should speed up performance for users that have a large number of clusters (at least 1500 per endpoint)

Right now the cluster list only gets the next available page so the page requests are sequential so the next page request only is fired when the immediate previous page request has been returned.