This refers to some resource that can return it's own "last changed" timestamp. It is used
in dependency lists for a generated resource where if a dependency changes the generated
data needs to be regenerated.
Return the current last modification time. This must return the ACTUAL modification time of the resource; the time
returned by this call will be compared with the time that the resource was last used (stored somewhere else) to decide if this
resource has changed in the meantime.
This call must return -1 for a resource that does not exist - because non-existence is a valid caching criteria too!