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

The aggregators are defined in src/storage/aggregator.h and are used during Put requests. Examine the current mutex usage and the concurrent access patterns. Research third-party concurrent hashmap libraries or atomic operations suitable for C++. A successful change will maintain thread safety while reducing memory overhead, and likely requires benchmarking.

Written by the indexing model from the issue text.