Add a Consumer API without groups

[dasch]

The Kafka Offset API allows un-grouped consumers. It may make sense for some use cases.

From the protocol spec:

Note that when this API is used for a "simple consumer," which is not part of a consumer group, then the generationId must be set to -1 and the memberId must be empty (not null). Additionally, if there is an active consumer group with the same groupId, then the commit will be rejected (typically with an UNKNOWN_MEMBER_ID or ILLEGAL_GENERATION error).

The thing is, for offset commits to work you still need a group id. I'm not sure what a nice API would look like.

[dasch]

Added an experimental API to master:

kafka = Kafka.new(...)

kafka.each_message(topic: "greetings") do |message|
  ...
end

It doesn't do checkpointing – I'm not sure if it makes sense to add it, as you might as well use a consumer group at that point.