change: use radixtree as default router.

[membphis]

I think we can use new router radixtree to replace r3 router, there are two reasons for this:

1. The compilation of r3 has higher requirements on the environment, and it is easy to install unsuccessfully.
2. The advantage of radixtree is that it supports full matching or deepest prefix matching, making it easier to get started, this is also more similar to routing within k8s.

准备把默认路由引擎由 r3 替换成 radixtree ，主要出于两点考虑：

1. r3 的编译对环境要求比较高，很多同学安装开发环境在这里卡壳。
2. radixtree 相比 r3 的优势是支持完整匹配和最深前缀匹配，从入门难度来说更简单，与 k8s 体系下路由策略更吻合。

[moonming]

please add more examples

[membphis]

please add more examples

https://github.com/iresty/apisix/blob/master/doc/architecture-design-cn.md#router

[membphis]

fixed #419