This commit introduces the changes discussed in PR #664:
* Remove `OffsetsProvider`: We only need offsets to be retrieved from
the execution context in a restart scenario
* Make the reader inherit from `AbstractItemStreamItemReader` instead
of `AbstractItemCountingItemStreamItemReader`. With Kafka offsets, item
counting does not make sense
* Remove `assignTimeoutProvider`. A configurable poll timeout with
a default value is enough.
* Update the constructor of the reader to accept consumer properties,
a topic name and a list of partitions instead of leaking Kafka APIs like
`TopicPartition` and `ConsumerFactory`.
Resolves BATCH-2764
b19998a40a