[BUG] OOM when use sentinel

## Issue Description

Type: *bug report*

When i running my app in prd for period time, it will cause oom.

The dump file:

### Describe what happened
OOM

### Describe what you expected to happen

### How to reproduce it (as minimally and precisely as possible)

1.
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### Tell us your environment
centos
sentinel 1.8.2

### Anything else we need to know?
Maybe it's related to async request like `DeferredResult`