Make sure do not update checkpoint ts when sink is stopping

[Rustin170506]

What did you do?

tiflow/cdc/sinkv2/tablesink/table_sink_impl.go

Line 133 in 4d31b61

e.state.Store(state.TableSinkStopping)

In this code, it might cause us to update checkpoints when the sink is stopping.

For example:

1. set state is stopping
2. drop the data in the backend sink (Concurrent with 1)
3. advance the checkpoint ts (Concurrent with 1)
4. try to close

What did you expect to see?

Do not update checkpoint ts.

What did you see instead?

Do update checkpoint ts.

Versions of the cluster

TiCDC version (execute cdc version):

master