[FLINK-37278] Optimize regular schema evolution topology's performance

[yuxiqian]

This closes FLINK-37278.

Currently, regular SE topology uses the following process to drain existing DataChangeEvents in the pipeline:

1. SchemaOperator ("client") emits FlushEvent to downstream.
2. The "client" keeps polling the SchemaCoordinator ("server") with 1-second interval.
3. The "server" rejects all requests from clients until it has collected enough FlushSuccessEvent notifications from Sink.

As a result, all schema change requests will took at least 1 second to finish, after at least one polling interval.

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This PR replaces the polling code with maintaining a pending schema change request queue, where SchemaCoordinator could manage all pending clients and effectively blocking them from handling upstream events. Schema evolution process could start immediately after FlushSuccessEvent got reported, needless to wait for polling requests from clients.

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With this change, time consumption of testRegularTablesSourceInMultipleParallelism test case has been reduced from ~6 minutes to ~50 seconds.

[yuxiqian]

Would @hiliuxg like to take a look?