whisper: get wnode to work with v6 (#16051)

The bulk of the issue was to adapt to the new requirement
that a v6 filter has to either contain a symmertric key or
an asymmetric one.

This commits revert one of the fixes that I made to remove
a linter warning: unexporting NewSentMessage. This is not
really a problem as I have a cleanup in the pipe that will
solve this issue.

whisper/whisperv6/envelope_test.go

@@ -45,7 +45,7 @@ func TestEnvelopeOpenAcceptsOnlyOneKeyTypeInFilter(t *testing.T) {
 
 	mrand.Read(params.Payload)
 
-	msg, err := newSentMessage(&params)
+	msg, err := NewSentMessage(&params)
 	if err != nil {
 		t.Fatalf("failed to create new message with seed %d: %s.", seed, err)
 	}