ext-plugin启用后，keepalive不生效

[hauerwu]

ext-plugin启用后，在ext-plugin-post-resp.lua里的before_proxy函数里会直接调用http库向upstream发起访问获得返回并关闭连接，这导致非ext-plugin能使用的keepalive连接池在ext-plugin里无法生效，请问有什么解决方案吗？是否要自己在lua里实现一个keepalive连接池？：

`function _M.before_proxy(conf, ctx)
local http_obj = http.new()
local res, err = get_response(ctx, http_obj)
if not res or err then
core.log.error("failed to request: ", err or "")
close(http_obj)
return 502
end
ctx.runner_ext_response = res

core.log.info("response info, status: ", res.status)
core.log.info("response info, headers: ", core.json.delay_encode(res.headers))

local code, body = ext.communicate(conf, ctx, name, constants.RPC_HTTP_RESP_CALL)
if body then
    close(http_obj)
    -- if the body is changed, the code will be set.
    return code, body
end
core.log.info("ext-plugin will send response")

-- send origin response, status maybe changed.
err = send_response(ctx, res, code)
close(http_obj)

if err then
    core.log.error(err)
    return not ngx.headers_sent and 502 or nil
end

core.log.info("ext-plugin send response succefully")

end`