Seeking to a specific offset for a partition

The Kafka consumer client has the functionality to seek to a specific offset for a partition--see [Controlling The Consumer's Position](https://kafka.apache.org/0100/javadoc/org/apache/kafka/clients/consumer/KafkaConsumer.html).

The current parallel consumer state does not allow seeking to a specific offset. We're looking to see if it is possible to do add to this library for us to be able to re-read messages if necessary.

Start with the Kafka Consumer documentation linked in the issue, especially the position and seek behavior, then inspect how the current parallel consumer represents partition state. Define what API and state changes are needed to seek and re-read messages, and verify the expected behavior against Kafka's consumer semantics.

Written by the indexing model from the issue text.