Rebalance listener correctness

[nikitapecasa]

This PR:

• adds a new way of describing rebalance listener (RebalanceListener1) via new subscribe method in Consumer
• which gives a correct behaviour of consumer methods executed from within RebalanceListener

def subscribe(topics: Nes[Topic], listener: RebalanceListener1[F]): F[Unit]

RebalanceListener1 callbacks are executed on current thread where consumer.poll/close/unsubscribe is running in a blocking fashion

example of new API usage:

class SaveOffsetsOnRebalance[F[_]: Applicative] extends RebalanceListener1[F] {
  import RebalanceCallback._
  def onPartitionsAssigned(partitions: Nes[TopicPartition]) =
    for {
      // read the offsets from an external store using some custom code not described here
      offsets <- lift(readOffsetsFromExternalStore[F](partitions))
      a       <- offsets.toList.foldMapM { case (partition, offset) => seek(partition, offset) }
    } yield a
  def onPartitionsRevoked(partitions: Nes[TopicPartition]) =
    for {
      positions <- partitions.foldM(Map.empty[TopicPartition, Offset]) {
        case (offsets, partition) =>
          for {
            position <- position(partition)
          } yield offsets + (partition -> position)
      }
      // save the offsets in an external store using some custom code not described here
      a <- lift(saveOffsetsInExternalStore[F](positions))
    } yield a
  // do not need to save the offsets since these partitions are probably owned by other consumers already
  def onPartitionsLost(partitions: Nes[TopicPartition]) = empty
}

List of allowed KafkaConsumer methods available via RebalanceCallback (as agreed with @t3hnar):

- assign
+ assignment
+ beginningOffsets
- close
- commitAsync
+ commitSync
+ committed
+ endOffsets
+ groupMetadata
+ listTopics
- metrics
+ offsetsForTimes
+ partitionsFor
- pause
+ paused
- poll
+ position
- resume
+ seek
+ seekToBeginning
+ seekToEnd
- subscribe
+ subscription
- unsubscribe
- wakeup