Fixes GB#621112: Resources not deleted from store when files removed from removable media
Fixes GB#621112: Resources not deleted from store when files removed from removable media

* As soon as mtime cache is updated with the contents of a given directory,
check if any element in the mtime cache no longer exists in the FS, and remove
it from the store if so.

outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker