Fix continuos loop of update checks if the refresh cache interval is 0 If the interval for refreshing the cache is set to 0 each check for available updates refreshes the cache and causes an "updates-changed" signal to be emitted after a short delay which in turn triggers another check for updates. The "updates-changed" signal does not actually mean that the number of available updates has changed but rather that the package metadata cache has been invalidated. Fix this by blocking the "updates-changed" signal handler from when a periodic or irregular check is started until 4 seconds after it has finished. This is necessary since the signal will be delivered with a delay after a transaction has been completed. In addition prevent checks for updates triggered by the "updates-changed" signal from refreshing the cache even the interval is 0 since the emission of the signal implies that the cache has just been refreshed.