Use a better cache strategy for statements_summary

[crazycs520]

Feature Request

Is your feature request related to a problem? Please describe:

Currently, statements_summary_history use LRU cache strategy to cache recent 200 statements and each statement with 24 histories by default.

The problem is, the history statements can be evicted by the new statement, so the variable tidb_stmt_summary_history_size (which default value is 24) doesn't mean STATEMENTS_SUMMARY_HISTORY can alway provide the last 24 statements history.

I hope we can find another cache strategy for statements_summary and also don't have to pay too much memory.

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: