Paxos protocol relies on replicas having a state that persists over
crashes/restarts. This patch defines such state and stores it in the
database itself in the paxos table to make it persistent.
The stored state is:
in_progress_ballot - promised ballot
proposal - accepted value
proposal_ballot - the ballot of the accepted value
most_recent_commit - most recently learned value
most_recent_commit_at - the ballot of the most recently learned value
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