[Issue 749]Fix panic caused by flushing current batch with an incorrect internal function.

[shileiyu]

…rrentBatches if the associated batch builder contains multi batches(KeyBasedBatchBuilder).

Doing transparent forwarding could centralized the branching logic at only one place.

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Fixes #749

Motivation

Eliminating multiple identical patches, which branch to the correct internal function by checking the type of associated batch builder.

Modifications

Calling internalFlushCurrentBatch will be forwarded to internalFlushCurrentBatches if the associated batch builder contains multiple batches(e.g. KeyBasedBatchBuilder).

Verifying this change

• Make sure that the change passes the CI checks.

This change is already covered by existing integration tests.

Does this pull request potentially affect one of the following parts:

None

Documentation

Not required

[shileiyu]

Rebased over the top of ci fixing (2.8.2)

[shileiyu]

Could someone trigger the CI procedure again?

[wolfstudy]

@shileiyu Greate jobs! Can we add unit test to cover this case?