[WIP] Add new RedisRepository

[Namoshek]

This PR re-introduces the RedisRepository deleted by #33, but with the new Repository interface and slightly different logic.

The new implementation utilizes hash maps for pending messages instead of sets. Most operations only involve one call to the Redis database, since we can use commands which terminate gracefully on failure (which we can then forward as exception or negative result to the caller).

Todos:

• [ ] Test implementation

This PR superseeds #40 which was based on an old branch.

[sonarcloud[bot]]

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