Add grpc retry (#1160)

* Add grpc retry

* Update grpc retry

* Add tests

* Fix a bug

* Add api && some tests

* Add comment

* Add double check

* Add server retry quota

* Update optimize code

* Fix bug

* Update optimize code

* Update optimize code

* Fix bug

core/retry/backoff/backoff_test.go

@@ -0,0 +1,17 @@
+package backoff
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+	"time"
+)
+
+func TestWaitBetween(t *testing.T) {
+	fn := Interval(time.Second)
+	assert.EqualValues(t, time.Second, fn(1))
+}
+
+func TestExponential(t *testing.T) {
+	fn := Exponential(time.Second)
+	assert.EqualValues(t, time.Second, fn(1))
+}