schedule,shared: move concurrency 429 limits into scheduler code (#849)

- make concurrency limiting the scheduler.Scheduler's responsibility
- eliminate the separate concurrency limit middleware 
- move concurrencyLimit logic into scheduler.FIFO to maintain backwards compatibility
- add HTTPError from #834 

Updates #834

internal/router/base_test.go

@@ -29,10 +29,10 @@ func (s *stubPlanner) OnSwapStart(string, []string)          {}
 func newTestBase(t *testing.T, processes map[string]process.Process, planner scheduler.Swapper) *baseRouter {
 	t.Helper()
 	conf := config.Config{HealthCheckTimeout: 5}
-	b := newBaseRouter("test", conf, processes, logmon.NewWriter(io.Discard),
-		func(name string, logger *logmon.Monitor, eff scheduler.Effects) scheduler.Scheduler {
-			return scheduler.NewFIFO(name, logger, planner, conf.Routing.Scheduler.Settings.Fifo, eff)
-		})
+	b, err := newBaseRouter("test", conf, processes, logmon.NewWriter(io.Discard), planner)
+	if err != nil {
+		t.Fatalf("newBaseRouter: %v", err)
+	}
 	b.testProcessed = make(chan struct{}, 64)
 	go b.run()
 	t.Cleanup(func() {