The error in the `KinesisMessageDrivenChannelAdapter.ShardConsumer.processTask()`
might be also thrown just direct from the `amazonKinesis.getRecords(getRecordsRequest)`.
If it happens first time, the `ShardCheckpointer` is not initialized with sequence numbers.
Therefore, a condition `highestSequence.equals(lastCheckpoint)` may lead to NPE.
* Rework the logic in the `KinesisMessageDrivenChannelAdapter.ShardConsumer.rewindIteratorOnError()`
to deal with a `null` for `this.checkpointer.getHighestSequence()` and reuse the current `shardIterator`
in the next request if no any commits happened.
* Remove `ShardCheckpointer.firstSequenceInBatch` since this is exactly a meaning of `shardIterator` representation.
Related to: https://github.com/spring-projects/spring-integration-aws/issues/223
d7a2dd590d