Kafka cluster sharding support with external sharding allocation

[seglo]

Summary

Provides several convenience setup functions to use a Kafka consumer group aware Akka cluster external sharding allocation strategy. What this means in simple terms is that Akka cluster entity shards will be co-located on akka cluster nodes with a Kafka Consumer's assigned partitions.

Fixes: #1074

Features

• Retrieve an Akka cluster sharding ShardingMessageExtractor for enveloped or non-enveloped entities.
• Automatically determine the number of shards to use in the ShardingMessageExtractor create given a topic.
• Manually assign the number of partitions in the ShardingMessageExtractor.
• Use the same Apache Kafka Murmur2 hashing function that the KafkaProducer DefaultPartitioner uses to make sure consumed messages are routed to the correct entity.
• Retrieve an Alpakka Kafka rebalance listener actor that can be used to automatically update the akka cluster external shard allocation mappings when a Kafka consumer group rebalance occurs.

For a sample usage see this WIP Akka Samples PR akka/akka-samples#192

TODO

• implementation
• docs

[ennru]

Looking forward to the docs, how should they integrate the Akka Sample?

[chbatey]

Looks good. My only comment is regarding APIs that combine classic and typed akka