Only way to be sure that plugins don't accidentally respond to onion_message
sent via reply path from another message (which would potentially leak our
identity!).
To quote BOLT #7 (Onion Messages) in the offers PR:
```markdown
The reader:
- MUST ignore any message which contains a `blinding` which it did not expect, or does not contain
a `blinding` when one is expected.
...
`blinding` is critical to the use of blinded paths: there are various
means by which a blinded path is passed to a node. The receipt of an
expected `blinding` indicates that blinded path has been used: it is
important that a node not accept unblinded messages when it is expecting
a blinded message, as this implies the sender is probing to detect if
the recipient is the terminus of the blinded path.
Similarly, since blinded paths don't expire, a node could try to use
a blinded path to send an unexpected message hoping for a response.
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
695a8bd868