Unified timer framework in TiDB

[lcwangchao]

There are many requirements for scheduling some jobs periodically in TiDB. For example, TTL needs to schedule a job to delete expired rows every several hours. The implementations look similar in these features, that is: schedule a background "watcher" task in the cluster to check the timer's condition. Once the timer is up to time, then schedule a new job to do something. The "watcher" task will also persist the timer's progress (such as the last job's execution time) to avoid a duplicated job's schedule.

It's better to provide a unified interface to all features with the above requirement.

Subtasks

• define user models/interfaces of timer framework #43836
• define store interfaces of timer framework #43938
• implement TimerGroupRuntime to trigger timer events in background #44175
• Implement table based TimerStore #44445
• Support set Tags for timer in timer framework #44603
• Use RW Lock for timer's memory store #44720
• Support trigger event manually for timer framework #45009
• Add system table mysql.tidb_timers to manage all timers in tidb #45128
• Add more tests for 'takeSession' and 'runInTxn' in timer table store #45527
• Add metrics to timer framework #45610
• Support cron syntax for timer framework #46330
• timer framework should auto recover when some background loop panic #46411
• Add README.md for timer framework #46857
• provide tidb level timer runtime registry #46856