* optimize: Use lru to avoid resolving IP addresses repeatedly .
Cached the global rules to `ctx` .
* optimzie: used a longer time interval for etcd and flush access log.
* optimize: return upstream node directly if the count is 1 .
* optimize: avoid to cache useless variable.
feature: support dynamic upstream in plugin.
here is a mini example in `access` phase of plugin:
```lua
local up_conf = {
type = "roundrobin",
nodes = {
{host = conf.upstream.ip, port = conf.upstream.port, weight = 1},
}
}
local ok, err = upstream.check_schema(up_conf)
if not ok then
return 500, err
end
local matched_route = ctx.matched_route
upstream.set(ctx, up_conf.type .. "#route_" .. matched_route.value.id,
ctx.conf_version, up_conf, matched_route)
return
```
