Many files problem on OSX Sierra

It seems like there is a problem with fsevents when dealing with a large amount
of files on OSX sierra this added test fails reliably on mac Macbook Pro, with
the error

```
2017-01-17 22:52 filemonitor.test[69205] (FSEvents.framework)
FSEventStreamStart: register_with_server: ERROR: f2d_register_rpc()
=> (null) (-22)
```

This more or less reporduces #589 #605 #607 on the basis of this being related
to many files the fix would be to not monitor files individualy on OSX but
rather monitor directories and filter client side, this should reduce the number
of files / directories to be monitored.

go/filemonitor/filemonitor_test.go

@@ -14,8 +14,8 @@ import (
 	"github.com/burke/zeus/go/filemonitor"
 )
 
-func writeTestFiles(dir string) ([]string, error) {
-	files := make([]string, 3)
+func writeTestFiles(dir string, count int) ([]string, error) {
+	files := make([]string, count)
 
 	dir, err := filepath.EvalSymlinks(dir)
 	if err != nil {
@@ -35,14 +35,58 @@ func writeTestFiles(dir string) ([]string, error) {
 	return files, nil
 }
 
+func TestFileMonitorManyFiles(t *testing.T) {
+	count := 5000
+	dir, err := ioutil.TempDir("", "zeus_test_many_files")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	files, err := writeTestFiles(dir, count)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	fm, err := filemonitor.NewFileMonitor(filemonitor.DefaultFileChangeDelay)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer fm.Close()
+
+	watched := files[0:count]
+	for i, file := range watched {
+		err = fm.Add(file)
+		if err != nil {
+			t.Fatalf("%d, %v", i, err)
+		}
+	}
+
+	changes := fm.Listen()
+
+	// Without a short sleep we get notified for the original
+	// file creation using FSEvents
+	time.Sleep(20 * time.Millisecond)
+
+	for i, file := range files {
+		if err := ioutil.WriteFile(file, []byte("baz"), 0); err != nil {
+			t.Fatalf("%d: %v", i, err)
+		}
+	}
+
+	if err := expectChanges(changes, watched); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestFileMonitor(t *testing.T) {
 	dir, err := ioutil.TempDir("", "zeus_test")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(dir)
 
-	files, err := writeTestFiles(dir)
+	files, err := writeTestFiles(dir, 3)
 	if err != nil {
 		t.Fatal(err)
 	}