Remove or hide `KeyStateOf` and `KeyState`

[rtar]

trafficstars

Currently, requirement to define KeyStateOf breaks a nice nesting pattern of flows: KafkaFlow -> ConsumerFlow -> PartitionFlow -> KeyFlow. I.e. there is KeyState instead of KeyFlow and KeyFlow is hidden in implementation.

The idea is to refactor this part a bit, may be do some renaming / removing of the code to restore the pattern in question.