ISSUE-749: Panic occurred when a producer associated with a KeyBasedBatchBuilder.

[sijie]

Original Issue: apache#749

---

Function internalFlushCurrentBatch, this line of code, should not be invoked if the batch builder associated with the producer is a multi batches container.

Potential Fix:
Using internalFlushCurrentBatches instead, if the associated batch builder is a multi batches container.

		if p.batchBuilder.IsMultiBatches() {
			p.internalFlushCurrentBatches()
		} else {
			p.internalFlushCurrentBatch()
		}

[shileiyu]

PR: apache#750
CI passed on 2.8.2.