request help: Do not use keepalive when connecting etcd v3

[gxthrj]

Now etcd v3 uses keepalive. When the user has a proxy between APISIX and etcd, and the proxy setting timeout is lower than the time of updating connection in connection pool, it will cause APISIX to report an error failed to fetch data from etcd: closed.

Ref: https://github.com/api7/lua-resty-etcd/blob/master/lib/resty/etcd/v3.lua#L114