clientv3: sends keepalive reqs immediately after lease keep alive stream reset

when lease client reset lease keep alive stream, sendKeepAliveLoop() should send out keep alive reqs immediately instead of waiting for 500ms.

clientv3/lease.go

@@ -452,16 +452,6 @@ func (l *lessor) deadlineLoop() {
 // sendKeepAliveLoop sends LeaseKeepAliveRequests for the lifetime of a lease stream
 func (l *lessor) sendKeepAliveLoop(stream pb.Lease_LeaseKeepAliveClient) {
 	for {
-		select {
-		case <-time.After(500 * time.Millisecond):
-		case <-stream.Context().Done():
-			return
-		case <-l.donec:
-			return
-		case <-l.stopCtx.Done():
-			return
-		}
-
 		var tosend []LeaseID
 
 		now := time.Now()
@@ -480,6 +470,16 @@ func (l *lessor) sendKeepAliveLoop(stream pb.Lease_LeaseKeepAliveClient) {
 				return
 			}
 		}
+
+		select {
+		case <-time.After(500 * time.Millisecond):
+		case <-stream.Context().Done():
+			return
+		case <-l.donec:
+			return
+		case <-l.stopCtx.Done():
+			return
+		}
 	}
 }