memory usage of analyze is not efficiently controlled by rate-limit

## Enhancement
Currently, analyze job will send requests to TiKV to fetch sample data with 30 currency which is 2*defaultConcurrency(15). In some cases, when sample-rate or data volume is large, analyze easily consumes much of the server memory, even oom, impacting user queries. So, the memory usage of analyze should be controlled well with requests' rate-limit.