fileutil: update watcher to use fsnotify and polling (#3663)

* fileutil: update watcher to use fsnotify and polling

* raise timeout

* maybe fix

internal/chanutil/merge_test.go

@@ -0,0 +1,37 @@
+package chanutil
+
+import (
+	"sort"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMerge(t *testing.T) {
+	ch1, ch2, ch3 := make(chan int), make(chan int), make(chan int)
+	go func() {
+		for _, i := range []int{1, 2, 3} {
+			ch1 <- i
+		}
+		close(ch1)
+	}()
+	go func() {
+		for _, i := range []int{4, 5, 6} {
+			ch2 <- i
+		}
+		close(ch2)
+	}()
+	go func() {
+		for _, i := range []int{7, 8, 9} {
+			ch3 <- i
+		}
+		close(ch3)
+	}()
+	out := Merge(ch1, ch2, ch3)
+	var tmp []int
+	for item := range out {
+		tmp = append(tmp, item)
+	}
+	sort.Ints(tmp)
+	assert.Equal(t, []int{1, 2, 3, 4, 5, 6, 7, 8, 9}, tmp)
+}