Extract reusable logic from OffsetFetcher

The OffsetFetcher is internally used by the KafkaConsumer to fetch offsets, validate and reset positions. 

For the new consumer based on a refactored threading model, similar functionality will be needed by the ListOffsetsRequestManager component. 

This task aims at identifying and extracting the OffsetFetcher functionality that can be reused by the new consumer implementation.