Sql. Fix invalid inflight transaction tracking.

PartitionScanSubscription may unexpectedly trigger `inflightBatchRequestTracker.onRequestEnd()` twice in case of batch processing error. in `scanBatch` method.

The first call is expected and happens on happy-path after getting a successful response.
But if batch processing failed with error (e.g. `subscriber::onNext`), then we get into unhappy-path and trigger `inflightBatchRequestTracker.onRequestEnd()` once again, which violates the contract.