Limit number of simultaneous HTTP requests

This is intended to prevent DNS errors while updating a large number of
sources.
Corrected argument passing for debounced functions
Increase debounce period before generating data

This reduces number of time the data is generated when browsing the web
page sends multiple 'mark as read' requests. May increase more in the
future.
Pick up changes to subscriptions while active

Restarting the program is no longer required to detect new or changed
subscriptions.