Atomic storage & In-memory cache for state

[AbyelT]

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• [X] You ran rustfmt on the code base before submitting (on latest nightly with rustfmt support)
• [X] You reference which issue is being closed in the PR text (if applicable)
• [ ] You updated the OmniPaxos book (if applicable)

Issues

Fix #40

Breaking Changes

• SequencePaxos reads from in-memory cache struct for state
• Operations that write to storage are Atomic

Other Changes

• Added CachedState for internal in-memory state storage
• Struct implementing Storage trait now returns Result for all operations