lib/model: Add global request limiter (fixes #6302) (#6303)

This adds a new config with the simple and concise name
maxConcurrentIncomingRequestKiB. This limits how many bytes we have "in
the air" in the form of response data being read and processed.

After some testing I think that not having this limiter is seldom a
great idea and thus I propose a default value of 256 MiB for this new
setting.

I also refactored the folder IO limiter to be a model/folder attribute
instead of a package global.

lib/model/model_test.go

@@ -3451,3 +3451,31 @@ func TestDeviceWasSeen(t *testing.T) {
 		t.Error("device should have been seen now")
 	}
 }
+
+func TestNewLimitedRequestResponse(t *testing.T) {
+	l0 := newByteSemaphore(0)
+	l1 := newByteSemaphore(1024)
+	l2 := (*byteSemaphore)(nil)
+
+	// Should take 500 bytes from any non-unlimited non-nil limiters.
+	res := newLimitedRequestResponse(500, l0, l1, l2)
+
+	if l1.available != 1024-500 {
+		t.Error("should have taken bytes from limited limiter")
+	}
+
+	// Closing the result should return the bytes.
+	res.Close()
+
+	// Try to take 1024 bytes to make sure the bytes were returned.
+	done := make(chan struct{})
+	go func() {
+		l1.take(1024)
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Error("Bytes weren't returned in a timely fashion")
+	}
+}