Add wrapper for Metadata requests

[ennru]

The new feature to request Kafka Metadata from the internal KafkaConsumerActor (see #491) would be even nicer with thin wrappers (javadsl and scaladsl) hiding the ask calls and mapping to the correct response types.

[2m]

The wrapper should also have Scala and Java DSLs with appropriate conversions between Java and Scala types and collections.

[jewertow]

I will take care of it.

[jewertow]

I started working on it but I wonder what return types do expect from the wrapper, e.g. in the case of EndOffsets the wrapper should return Future[Long] or Future[Option[Long]]?

[ennru]

With the current GetEndOffsets(Set[TopicPartition]) the response is EndOffsets(Map[TopicPartition, Long]). So I'd expect Future[Map[TopicPartition, Long]].

[ennru]

Implemented in #497 by @jewertow