[TASK] Be a bit less eager to remove Asset files

This should help improve VHS's performance greatly on sites which use a lot of Assets; before this change, VHS would remove Asset files on any type of cache clearing and would not hesitate to remove Asset files multiple times in one Request. After this change removal of files is only ever allowed once per Request and is only done when clearing _all_ caches in the backend, ensuring it doesn't happen too often (e.g. whenever an editor saves a page or clears the page content cache).
Merge pull request #250 from NamelessCoder/cache

[TASK] Be a bit less eager to remove Asset files