feat: resty.limit.count support new shared dict incr function

[windmgc]

Hi,
The aim of this pr is to make resty.limit.count's incoming function using the latest type of shared dict's incr function, which supports setting expire time when increasing an unexisting key, and it is also compatible with the old version of OpenResty, just as mentioned in #34 (comment).

I'm doing some pressure tests recently to test the stability of lua-resty-limit-traffic module and found some problems under high volume(possibly due to the concurrent situation and two separate atomic operation(incr and expire) calls in incoming, see #23 (comment) and #47 (comment)). However, I found that when I'm using the latest incr function which supports expire time(which makes the two operation combines into an atomic one(done within the same lock, see https://github.com/openresty/lua-nginx-module/blob/3f33dd862a3d8539b96dda09b624c89712cbe0c8/src/ngx_http_lua_shdict.c#L1889), the problem hardly occurs. So I think it is perhaps a good idea to use incr with expire time arg instead of separate them into two operations.

[doujiang24]

This way should be better:

local function incoming_new
local function incoming_old
_M.incomming = incr_support_init_ttl and incoming_new or incoming_old

[windmgc]

@doujiang24 fixed, please review again, thanks