PERF: message_bus will be deferred by server when flooded

The message_bus performs a fair amount of work prior to hijacking requests
this change ensures that if there is a situation where the server is flooded
message_bus will inform client to back off for 30 seconds + random(120 secs)

This back-off is ultra cheap and happens very early in the middleware.

It corrects a situation where a flood to message bus could cause the app
to become unresponsive

MessageBus update is here to ensure message_bus gem properly respects
Retry-After header and status 429.

Under normal state this code should never trigger, to disable raise the
value of DISCOURSE_REJECT_MESSAGE_BUS_QUEUE_SECONDS, default is to tell
message bus to go away if we are queueing for 100ms or longer

spec/integration/rate_limiting_spec.rb

@@ -14,6 +14,31 @@ describe 'rate limiter integration' do
     RateLimiter.disable
   end
 
+  it "will rate limit message bus requests once queueing" do
+    freeze_time
+
+    global_setting :reject_message_bus_queue_seconds, 0.1
+
+    post "/message-bus/#{SecureRandom.hex}/poll", headers: {
+      "HTTP_X_REQUEST_START" => "t=#{Time.now.to_f - 0.2}"
+    }
+
+    expect(response.status).to eq(429)
+    expect(response.headers['Retry-After']).to be > 29
+  end
+
+  it "will not rate limit when all is good" do
+    freeze_time
+
+    global_setting :reject_message_bus_queue_seconds, 0.1
+
+    post "/message-bus/#{SecureRandom.hex}/poll", headers: {
+      "HTTP_X_REQUEST_START" => "t=#{Time.now.to_f - 0.05}"
+    }
+
+    expect(response.status).to eq(200)
+  end
+
   it "will clear the token cookie if invalid" do
     name = Auth::DefaultCurrentUserProvider::TOKEN_COOKIE