Issue description

{
"count": 4000,
"key": "server_addr",
"policy": "redis-cluster",
"redis_cluster_nodes": [],
"rejected_code": 500,
"time_window": 1
}

When the threshold not reached,a few requests are prevented by the limit-count plugin.
The founction incoming, ttl and incrby opreations are not atomic.
Maybe can use eval() to ensure it.

Environment

• apisix version : 2.0
• OS (cmd: uname -a): Linux 2020 x86_64 x86_64 x86_64 GNU/Linux
• OpenResty / Nginx version (cmd: nginx -V or openresty -V): openresty/1.19.3.1

Minimal test code / Steps to reproduce the issue

压测即可复现