Optimize concurrent access for aggregators

**Describe the feature you'd like**

The aggregators may be accessed concurrently. Now each of the aggregators is protected by its mutex. It is costly in terms of memory usage.

We should design a more efficient thread-safe method to access the aggregators.

Some possible directions:

1. introduce the implementation of concurrent hashmap from third-party libraries.
2. asynchronous update for the aggregators
3. introduce some `atomic` way to eliminate the `lock`

**Additional context**
pre-aggregators update during `Put` request

src/storage/aggregator.h

聚合器定义在 src/storage/aggregator.h 中，并在 Put 请求期间使用。检查当前的互斥锁使用情况和并发访问模式。研究适用于 C++ 的第三方并发哈希表库或原子操作。成功的更改将在保持线程安全的同时减少内存开销，并且可能需要进行基准测试。

由索引模型根据 Issue 内容生成。