Websocket: Fix race on DataHandler during shutdown

Previously we would encounter situations where shutdown would race and
fail TestConnectionMessageErrors, demonstrating that consumers might not
be told why their connection is closing.

* Do not drain DataHandler on dataMonitor shutdown
Consumers should be allowed to process whatever messages were in flight
prior to a socket shutdown

* Ensure all DataHandler messages are sent to ToRoutine before shutdown
This avoids a race where we'd read a message from DataHandler, but then
process a shutdown before trying to relay it. The relay is non-blocking
anyway, so we can trust that we'd pick up the Shutdown during the next
loop.

* Send errors to DataHandler before starting a shutdown
This just reduces the chance of a race on shutdown processing

exchanges/stream/websocket_test.go

@@ -326,8 +326,6 @@ func TestConnectionMessageErrors(t *testing.T) {
 	err = ws.Connect()
 	require.NoError(t, err, "Connect must not error")
 
-	ws.TrafficAlert <- struct{}{}
-
 	c := func(tb *assert.CollectT) {
 		select {
 		case v, ok := <-ws.ToRoutine:
@@ -345,6 +343,7 @@ func TestConnectionMessageErrors(t *testing.T) {
 		}
 	}
 
+	ws.TrafficAlert <- struct{}{}
 	ws.ReadMessageErrors <- errDastardlyReason
 	assert.EventuallyWithT(t, c, 2*time.Second, 10*time.Millisecond, "Should get an error down the routine")