Internal in-memory storage for OmniPaxos state

[haraldng]

Currently OmniPaxos reads directly from the user-provided Storage for variables such as the promised_round and decided_idx. This will be inefficient if Storage is storing these in persistent storage. Thus, OmniPaxos should have an internal storage struct that caches these values and reloads them from the persistent storage upon failure recovery (might be related to #22)