maintain status: number of slow writes&commits since mysqld start up

[jd-zhang]

Issue migrated from trac ticket # 783 www.kunlunbase.com

component: kunlun-storage | priority: major

2022-06-01 15:39:02: zhaowei@zettadb.com created the issue

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'slow' means the query is qualified to be logged in(not necessary logged, since slow logging may be off) slow log.

[jd-zhang]

2022-06-01 15:40:07: zhaowei@zettadb.com edited the issue description