feature(core): implement core.sleep

[membphis]

Use a small time interval (such as 1s) to complete a relatively long sleep behavior, so that the worker process can be shut down gracefully.

Here is the fake code(we may):

function sleep(sec)
    if sec <= 1 then
        ngx_sleep(sec)
        return
    end

    while true do
        if sec > 1 then
            ngx_sleep(1)
        end

        if exiting() then
            return
        end

        sec = sec - 1
        if sec < 1 then
            ngx_sleep(sec)
            return
        end
    end
end

[dabue]

please assign it to me.

[dabue]

I 'll add a new function sleep to the file apisix/core.lua, and replace all filed ‘sleep = ngx.sleep’ with 'sleep = require("apisix.core.sleep")'. Is that OK?

[membphis]

it is fine, let us do it ^_^