SimpleConfigReader.Reload, avoid double reloads

If for some reason we try to call Reload twice but the config file
hasn't changed, we don't want to load it again. Use stat() on the file
to cheaply avoid that situation.

We also return a boolean value, according to whether we have reloaded
the config or not.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>