This is useful when you're using a label to determine the receiver but
want to override other options such as `group_by`. Currently you'd have
to duplicate the matchers for the receivers to be able to do this.
`checkReceiver()` now returns no error if a receiver is empty and we add
a check to ensure that the root route has a receiver defined. I've added
a test for this.
This brings the `receiver` option into line with the other options
(`group_by`, `group_wait`, etc) in the sense that routes can now inherit
the receiver from the parent node.
From https://prometheus.io/docs/alerting/configuration/:
> A route block defines a node in a routing tree and its children. Its
> optional configuration parameters are inherited from its parent node
> if not set.
