When the value of a `dynamic` option is updated core Lightning
will make a JSON-RPC request to the plugin.
The documentation falsely mentioned the request has a method named
`setvalue. It appears it is actually called `setconfig`.
Changelog-Added: Plugins: plugins can now specify (unknown) even messages we should accept from peers.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>