Event store has a race around subscription checkpoint updates

[lidezhu]

What did you do?

In UpdateDispatcherCheckpointTs, dispatcher checkpoint is updated while subscription checkpoint is recalculated from all subscribers. Concurrent calls may observe stale values and generate out-of-order subscription state updates.

What did you expect to see?

No response

What did you see instead?

None

Versions of the cluster

Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

(paste TiDB cluster version here)

Upstream TiKV version (execute tikv-server --version):

(paste TiKV version here)

TiCDC version (execute cdc version):

(paste TiCDC version here)