KafkaBasedLog's consumer should always seek to beginning when starting

KafkaBasedLog expects that callers set the `group.id` for the consumer configuration, and does not itself set the `group.id` if the caller does not explicitly do so. However, KIP-289 changed the default for the `group.id` from a blank string to be null, which changes how KafkaBasedLog behaves when no `group.id` is set, and it actually deprecates and issues a warning when no `group.id` is specified.

When KafkaBasedLog starts up, it should always start from the beginning of the topic and consume to the end. The consumer's logic for where to start is always:

1. explicit seek
2. committed offset (skipped if group.id is null)
3. auto reset behavior

and currently Connect does not explicitly seek to the beginning and instead relies upon `auto.offset.reset=earliest`. However, if a `group.id` is specified ant there are committed offsets, then the consumer will start from the committed offsets rather than from the beginning. If a 'group.id' is not specified, then the auto reset behavior should work.

However, to avoid the warning and possible exception when no `group.id` is specified, KafkaBasedLog should always call consumer.seekToBeginning() during startup.