use time.NewTimer() to avoid possible memory leaks (prysmaticlabs#13800)

Co-authored-by: Preston Van Loon <pvanloon@offchainlabs.com>

beacon-chain/rpc/prysm/v1alpha1/validator/server.go

@@ -99,10 +99,15 @@ func (vs *Server) WaitForActivation(req *ethpb.ValidatorActivationRequest, strea
 		return status.Errorf(codes.Internal, "Could not send response over stream: %v", err)
 	}
 
+	waitTime := time.Duration(params.BeaconConfig().SecondsPerSlot) * time.Second
+	timer := time.NewTimer(waitTime)
+	defer timer.Stop()
+
 	for {
+		timer.Reset(waitTime)
 		select {
 		// Pinging every slot for activation.
-		case <-time.After(time.Duration(params.BeaconConfig().SecondsPerSlot) * time.Second):
+		case <-timer.C:
 			activeValidatorExists, validatorStatuses, err := vs.activationStatus(stream.Context(), req.PublicKeys)
 			if err != nil {
 				return status.Errorf(codes.Internal, "Could not fetch validator status: %v", err)