optimze: reduce the ngx.req.socket object size & reduce ngx.socket.udp object size.

[doujiang24]

Seems we only need to save the timeout values to the ngx_http_lua_socket_tcp_upstream_t u for the connected request socket. And the udp socket don't have to save the key for keepalive.

I hereby granted the copyright of the changes in this pull request to the authors of this lua-nginx-module project.

[mergify[bot]]

This pull request is now in conflict :(