[ObjC] log when got GRPC_QUEUE_TIMEOUT event from completion queue

[HannahShiSFB]

This is for the tat task app sometimes crash with "Unrecognized completion type". However in obj-c wrapper, it is calling grpc_completion_queue_next with gpr_inf_future where it should never receive a timeout event. This PR add a log and reschedule another grpc_completion_queue_next for a GRPC_OP_COMPLETE or GRPC_QUEUE_SHUTDOWN event, which is similar to the gRPC C++ implementation