[Kinesis] Support the MultistreamTracker API.

[dluxhu]

PRELIMINARY PR, NOT READY TO MERGE YET

Add support for the new MultistreamTracker API.

This PR contains:

• A new field in ShardProcessorData: streamIdentifer. This way the consumers of CommittableRecord are able to determine which stream the record came from.
• Update for the checkpointing logic, now it takes the stream identifier into account, too when grouping the records, not hust the shard id.

Usage

There are minimal changes in the usage of KinesisSchedulerSource:

• KinesisSchedulerSource is now able to track multiple streams.
• checkpointRecordsFlow and checkpointRecordBatch now takes an optional maxShardCount parameter, because the maximal shard count can now be over the maximum amount of shards of one flow (500).

Note: this PR contains changes that are to be revertedn before merging to the Akka repository.

TODO

• Tests
• Java version
• Documentation