Better docs and unit tests for Storage

[haraldng]

The storage trait should have better descriptions, especially related to the indexes e.g., whether get_log_len should return only the number of real entries or be calculated based on the compacted index.

Furthermore, we currently only have tests for the storage in OmniPaxos. However, we should also have unit tests directly in omnipaxos_storage. This will make it easier for users to understand how to implement and test their own storage implementations.

[soruh]

Thank you! I think it would help to have tests available for users to test their own Storage implementations against. (Similar to what I attempted here: https://gitea.h.glsys.de/soruh/validate_omnipaxos_storage). This could be reused to test the implementations in omnipaxos_storage. If you think that's reasonable feel free the reuse what you want of that code. I would do this myself, but I obviously don't know what to test...