Add channel API

I try to add a new API about the channel (Codes are done, ready for pull request). It has the same characteristics as golang channel. You could create a new channel, with or without capacity, send messages to the channel, receive messages from the channel, close the channel, and do select upon multiple channel operations. You could share the channel between different request contexts.

Here are some examples:

Simple send and receive:

    content_by_lua_block {
        local ch = ngx.channel.new(1)
        local ok, err = ch:send("hello")
        if not ok then
            ngx.say("error")
        end
        local th = ngx.thread.spawn(function()
            local val, ok = ch:recv()
            ngx.say(ok, " : ", val)
        end)
        ngx.thread.wait(th)
    }

Select:

    content_by_lua_block {
        local ch1 = ngx.channel.new()
        local ch2 = ngx.channel.new()
        local th = ngx.thread.spawn(function()
            ngx.channel.select({
                {ch1, "recv", function(val, ok)
                    ngx.say(val, " : ", ok)
                end},
                {ch2, "send", "foobar"},
            })
        end)
        ch1:send(12)
        ngx.thread.wait(th)
    }

Select with default statement:

    content_by_lua_block {
        local ch1 = ngx.channel.new()
        local ch2 = ngx.channel.new()
        local th = ngx.thread.spawn(function()
            ngx.channel.select({
                {ch1, "recv"},
                {ch2, "send", "foobar"},
                {"default", function() ngx.say("default") end}
            })
        end)
        ch1:close()
        ngx.thread.wait(th)
    }

Are you interested in this API? Any suggestions are welcome.

No files, tests, or entry points are named. Start by comparing the requested Lua API examples with the module's existing public API and channel-related entry points, then clarify where the completed code and validation belong. Done requires agreed behavior for channel creation, send/receive, close, select, and sharing across request contexts.