plugin/pkg/uniq: add (#1733)

Spin this out the metrics package so we can use it in the health
one of well to fix some reload bugs.

plugin/pkg/uniq/uniq_test.go

@@ -0,0 +1,17 @@
+package uniq
+
+import "testing"
+
+func TestForEach(t *testing.T) {
+	u, i := New(), 0
+	u.Set("test", func() error { i++; return nil })
+
+	u.ForEach()
+	if i != 1 {
+		t.Errorf("Failed to executed f for %s", "test")
+	}
+	u.ForEach()
+	if i != 1 {
+		t.Errorf("Executed f twice instead of once")
+	}
+}