KafkaOffsetReader's consumers should not be in the same group

In `KafkaOffsetReader`, when error occurs, we abort the existing consumer and create a new consumer. In our current implementation, the first consumer and the second consumer would be in the same group, which violates our intention of the two consumers not being in the same group.

The cause is that, in our current implementation, the first consumer is created before `groupId` and `nextId` are initialized in the constructor. Then even if `groupId` and `nextId` are increased during the creation of that first consumer, `groupId` and `nextId` would still be initialized to default values in the constructor.

We should make sure that `groupId` and `nextId` are initialized before any consumer is created.