Fix dmarc reject (#260)

* Update data_post_headers documentation
We cannot reject at this stage, which is only there to alter headers.
Fix #258

* DMARC plugin: reject in data_post
Followup of #258: we cannot reject a connection during data_post_headers. So add a new hook in data_post to do the real rejection

* Add the dmarc_reject note on the transaction
It's probably possible to have one valid email after an invalid one using the same connection (even if not common)

plugins/dmarc

@@ -192,12 +192,12 @@ sub check_dmarc {
 
     $self->adjust_karma(-3);
     # Add a mark now so the data_post hook can do the real reject
-    $self->connection->notes('reject_dmarc', '1');
+    $transaction->notes('reject_dmarc', '1');
 }
 
 sub reject_dmarc {
     my ($self, $transaction) = @_;
     return $self->get_reject("failed DMARC policy")
-        if ($self->connection->notes('reject_dmarc'));
+        if ($transaction->notes('reject_dmarc'));
     return DECLINED;
 }