Set base URI, base URI is used when doing routing, in routing phase request
URI is used to route a request, while base URI is used to skip the
leadding part(base URI) of request URI.
That is, if comes a request with request URI a/b/c, then if you set base
URI to "a/b", only "/c" will be used in routing phase.
Note:
generally, you don't need to set this, Yaf will determine it automatically.