We used to need to do timed requeues in a few places in the reconcile logic,
and the easiest way to do that was to plumb reconcile.Result return values
around. But now we're purely event-driven, so the only thing we care about
is whether or not an error occurred.
Incidentally also fix a very minor bug where headless services would get
completely ignored, rather than reconciled into the correct state. This
shouldn't matter in practice because you can't transition from a headful
to a headless service without a deletion, but for consistency let's avoid
having a path that takes no definite action if a service of interest does
exist.
Updates #502.
Signed-off-by: David Anderson <danderson@tailscale.com>