NP, we can live with required `JsonMessageParser` as constructor argument.
I've fllowed here with similar solution from `MessageGroupStore.MessageGroupCallback`, where any `MessageGroupCallback` implementation will have dependency from `MessageGroupStore` too.

> (they have references to each other)

Can we make them as inner classes within `JsonInboundMessageMapper` ?

> feel free to close this PR 

Don't worry: I don't have enough permissions :smile_cat: