Feature request: dynamic listen/close socket in lua

Hi there,

I think many user need the new feature that we can dynamic listen or close a socket in the lua context, such as the discuss here, or some user' PR #1393 .

About the usage of this function, I have some ideas:

1. The usage should like this:

function accept_hook(socket)
     local b, err = sock:receive(8) -- like ngx.tcp.socket/ngx.udp.socket, it is a stream-typed cosocket
     if err then
         return
    end

    sock:send("welcome")
    ngx.exit() -- close the socket for tcp
end

local err = ngx.socket.listen("tcp://127.0.0.1:5050",  accept_hook) -- can not bind same address more than one time
if err then
    ngx.log(ngx.ERR, "listen failed")
    return
end

And support close the listen by the address, like this:

local err = ngx.socket.close("tcp://127.0.0.1:5050")
if err then
    ngx.log(ngx.ERR, "listen failed")
    return
end

2. ngx.socket.listen for TCP, will bind, listen and accept a socket, and put the socekt into event list, like ngx_event_accept, and for UDP, will bind and recvmsg for the socket, like [ngx_event_recvmsg], and put the socekt into event list(https://github.com/nginx/nginx/blob/master/src/event/ngx_event_udp.c#L32).

3. This should be like ngx.timer, can use in each phase. If run it in init_by_lua, all worker will bind same address by reuseport.

4. Different worker can bind same address in different time by reuseport.

5. When the worker exit, all listen will close automatic.

6. A new independent phase should be build(like timer, maybe name as lua_socket?), some function, such as ngx.req.* can not be use.

Start with the referenced NGINX entry points ngx_event_accept.c and ngx_event_udp.c, then inspect the existing ngx.timer and ngx.socket APIs in lua-nginx-module. Define how ngx.socket.listen and ngx.socket.close should work for TCP and UDP across worker phases, including init_by_lua and worker exit. Done means the behavior is implemented with coverage for binding, accepting or receiving, closing, reuseport, and lifecycle handling.