/tiflash/table/sync not cleared

[breezewish]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Add TiFlash replica to a table, then frequently truncating the table.

2. What did you expect to see? (Required)

Only one key in the /tiflash/table/sync prefix in PD etcd, if the table is not synced, or no key remains, if the replica sync is finished.

3. What did you see instead (Required)

Multiple keys are remained and uncleared in PD etcd. When there are too many keys left, there will be huge scan cost when querying for INFORMATION_SCHEMA.TIFLASH_REPLICA. This will affect other etcd queries and features rely on etcd, for example, DDLs.

4. What is your TiDB version? (Required)

master