@Nullable
    public static <T> TtlVertxHandler<T> get(@Nullable Handler<T> handler) {
        return get(handler, false, false);
    }
Factory method, wrap input {@link Handler} to {@link TtlVertxHandler}.

@param handler input {@link Handler}. if input is {@code null}, return {@code null}.
@return Wrapped {@link Handler}
@throws IllegalStateException when input is {@link TtlVertxHandler} already.
ttl-integrations/vertx3-ttl-integration/src/main/java/com/alibaba/ttl/integration/vertx3/TtlVertxHandler.java