feature: balancer_by_lua_* for stream subsystem

[doujiang24]

For now, only set_current_peer and set_more_tries(not respect proxy_next_upstream_tries) are provided.

1. for get_last_failure, there is only one fail status, so we do not need it?
2. for set_timeouts
   the connect_timeout only in ngx_stream_proxy_srv_conf_t,
   and it only support proxy_connect_timeout and proxy_timeout (no proxy_read_timeout and proxy_send_tiemout like http system does), so I suggest just mark it to do for now.
3. for set_more_tries
   Seems we need to patch to the nginx core to get the configured max retry num, so just not respect proxy_next_upstream_tries for now, marked it's todo.

The nginx stream system is growing, so I think it's better to not patch for the nginx base code.

This PR is basically from lua-nginx-module & learn much from @rshriram @splitice #30

[rshriram]

@doujiang24 I was just about to start addressing your comments. Looks like you beat me to the chase. Glad that you got these issues sorted out. Thank you.

[doujiang24]

@rshriram I needed it to build a dynamic tcp proxy system, and it's online now.
Also will you have a look at it? Thanks for your work too :)

[rshriram]

@doujiang24 can you possibly point me to your dynamic tcp proxy?
How did you overcome the lack of variable support in tcp subsystem?

[agentzh]

@rshriram I think the latest nginx core has got nginx variable support.

[rshriram]

@agentzh is there any way to combine the balancer_by_lua functionality with nginx features like max_fails, fail_timeout, max_conns (http://nginx.org/en/docs/stream/ngx_stream_upstream_module.html#server) ?

In the same vein, how do I combine lua-resty-upstream-healthcheck (http/tcp) with balancer_by_lua ?

[doujiang24]

@rshriram Sorry, there is no public code.
My dynamic upstreams is loaded by init_worker_by_lua + ngx.timer.at, and don't need variable on my side.

[dndx]

No longer needed.