Feature: Async Insert

[sundy-li]

Summary

Description for this feature.

With Async Insert, high rate of small inserts will be buffered by the server into batches.
This sacrifices tiny latency for better insert throughput and smaller block count in storage.

Configs may be introduced:

1. async_insert_wait_ms: time to wait to be grouped into batches for the single insert, defaults to 200ms.
2. async_insert_max_rows: the maximum rows of a single grouped batch to insert to storage, defaults to 100000 .

[fkuner]

/assignme