Admin page loads slowly

The admin  page for BackUpWordPress loads very slowly ( > 7s ) , I have identified the cause of this to be due to the fact that we are recalculating the total size of files on every page load because the transient gets cleared each time the `support` schedule object is instantiated.
@willmot  proposed to change the transient name to contain the backup type, so we can remove the cache clearing call from the `set_type` function that is called on every new object.