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/matrix_test.go

@@ -10,7 +10,6 @@ import (
 	"github.com/mostlygeek/llama-swap/internal/config"
 	"github.com/mostlygeek/llama-swap/internal/logmon"
 	"github.com/mostlygeek/llama-swap/internal/process"
-	"github.com/mostlygeek/llama-swap/internal/router/scheduler"
 )
 
 // newTestMatrix builds a Matrix router from supplied processes, bypassing
@@ -22,10 +21,10 @@ func newTestMatrix(t *testing.T, conf config.Config, expanded []config.ExpandedS
 		solver: newMatrixSolver(expanded, evictCosts),
 		logger: logger,
 	}
-	base := newBaseRouter("matrix", conf, processes, logger,
-		func(name string, l *logmon.Monitor, eff scheduler.Effects) scheduler.Scheduler {
-			return scheduler.NewFIFO(name, l, swapper, conf.Routing.Scheduler.Settings.Fifo, eff)
-		})
+	base, err := newBaseRouter("matrix", conf, processes, logger, swapper)
+	if err != nil {
+		t.Fatalf("newBaseRouter: %v", err)
+	}
 	base.testProcessed = make(chan struct{}, 64)
 	r := &Matrix{baseRouter: base}
 	go base.run()